I've been on both sides. Although I haven't shot that many weddings. My sister was getting married (this was before I decided to go "pro") and she wanted me to do video of the ceremony, so I brought the gear that I had at the time. (D7100, D3200, D40 + 18-140mm, 55-200, 50mm, 35mm) I set up for video and I did go around and take some snapshots, but I tried to stick with just candids and during posed photos I only took meta shots of the photographer taking photos of the people. (Where the photographer was in focus, but the people weren't.)
In hindsight, I think this was a big faux pa, but I was definitely just a hobbyist at the time and hadn't done any research on wedding photography or anything like that at all. In fact this was only like the second or third wedding I had ever attended. The photographer was a friend of my sister's and I stayed out of his way, but I could definitely understand if he was a bit irritated. If he had asked me to I would have just stuck to video and gotten out of his way. Being an enthusiast and taking my photography really seriously, I posted the photos the day after (like within 12 hours) with a custom monogram and everything. I think in this situation I was lucky, because the photographer was my sister's friend and it was my sister's wedding. But, in the future I will probably just bring my gear in the trunk (for worst case scenarios, plus I bring my gear with me everywhere. It's like clothing to me) and stick to a film camera during the wedding. I think in the end, it was also OK because he had a lot more diversity in his shots, he had all the posed photos and his gear was better than mine (plus I was part of the wedding party). My photos were also more of the rehearsal and rehearsal dinner. For that situation, I'll just have to feign ignorance, being just an enthusiast. At that wedding I actually didn't see a single other person with a DSLR and I was stuck on the balcony for most of it, so I don't think I really bothered him that much.
On the other hand, in my contracts now I do have a clause saying something along the lines that I'll be the only "pro" photographer and it's difficult for me to do my job if I have to compete with other people's flashes or if they get in the way of some of the shots during the ceremony, etc. But, at least from my experience that was far, far from the truth. I think 3 people in a 19 person wedding had DSLR's and were popping their on camera flash and off camera flashes. Honestly, it didn't bother me much, because I brought 4 cameras and had upgraded to a more "pro" set up. (D750 + D7100 + 24-70 + 70-200). I even offered to let them borrow some of my DX lenses like the 35mm 1.8, Tokina 12-24 or the 55-200, to play around with if they wanted to. In that case, it was obvious they were just enthusiasts and the older people enjoyed talking shop and film, while the younger people just liked to try out their new cameras. BUT, I can tell you that if someone came in with a D4 or D5 and was snapping shots at 10000 fps, it would make me a little bit upset. But at least for me. I'm a bit competitive, so as long as it didn't ruin my shots I would just push myself to diversify my photos and be more creative. (If anything I would just feel like trash if their photos were way better than mine)
In another situation I was doing an event in a chapel and like 3 different guys had 70-200 lenses and a handful of people had kit or shorter equivalents and were trying to take photos. It didn't bother me because I'm confident in my photos, I had two cameras with two lenses and a variety to choose from if I needed to change it up. Plus, I also knew that the guys standing in the back with kit lenses and entry level bodies were going to get prettty much nothing usable. Also, I would take some meta photos of them taking photos and then later on copy their exact position and take some more photos. (Got to cover those bases)
In your situation, I think it's really just chalked(chocked?) up to miscommunication. It's very possible that the main photog was told something and you were told something else. They could have just been told that "my second cousin will be taking some photos also, I think they have some experience with weddings". They may have interpreted that as if you would just be coming in with the similar level of gear as them and just taking some snaps here and there.
I think you did the right thing by backing off and letting them take care of the rest of the wedding. It's possible they were intimidated by your presence and I think it would be similar to if someone came in with a 3 person crew with strobe/studio lighting, a videographer and 2 x D5s's taking 9999 photos per second.
Obviously, since you're a photographer you're going to be proud of your photos and want to post them on your page. I think that's fine as long as you're not friends with the official photog. Maybe keep it on friends only? If you really want to just destroy this guy I would post them on public and tag everyone on them like right after he posts his, but you know.......
People are weird and can do weird things, I wouldn't stress too much about it. You could have done everything right and they could have known in advance about your level and everything, but it might have been their first wedding and the photos they were getting weren't coming out the way they wanted to. Or they could have just been stressed out. I wouldn't worry about it too much. Different things stress different people out and not everyone is easy to work with and not everyone can keep their cool.