I found it funny because he said he would like to make a video instructing how to stretch, tried to describe the method, and got some backlash with his description, reinforcing his idea that he needs to make a video. Think of the spongebob pajamas school picture day meme, that's the expression I saw him with.
I'll try to weigh in on your comment skills. I feel like I should because I used to be really rude unintentionally until I started being aware of how other people felt after talking with me, but I got better with some direction. So here goes.
Your phrasing is really blunt and impersonal, giving off the impression that you are being a dick.
Never ever ever start off a reply with "You're wrong." that just screams 'I'm an asshole'.
Black and white phrases like "Unless you x, you end up with y" are also very poor at reaching an audience, because immediately they will look for an alternative to x (and people are very good at rationalizing, so an alternative always arises), and since they found one that works for them, they will just assume that you are being arrogant and haven't considered what they have considered (which is ironic, but you can't change how people react to things)
Instead of talking about things like they are fact (even if they are, people are not good at being instructed on how they are doing things wrong, they tend to get emotional and defensive) talk about how experiences worked for you, and then encourage them to take the same path. Trump calls this "pulling the wet noodle". It's a social tactic that is often seen in sales etc. Ever try to push a wet noodle across a table? It breaks real quick. But you can easily pull it without breaking it. Same thing with ideas and conversation. If you try to push someone to your idea, the noodle will break. However, if you act on the power of suggestion you are "pulling" them toward your idea, making it easier for the person to accept. It's a pretty good metaphor.
If you are interested in improving your social skills, I would recommend looking into a couple different areas. Sales tactic books are pretty good because they break interaction down to a science, something people like you and me can easily relate to. Careful not to employ these tactics in real life, you don't want to be a walking talking salesman, but they can definitely give you some great hints as to what to do and what not to do. Another great area to look at is "How to win friends and influence people" I'm sure you have heard it talked about before. It isn't really a guide to making friends, but more of an instruction manual on how to handle people in ways that are beneficial to both parties. It's really good and worth re-reading. Another area you can look into is the pick up artist scene. It's a good place to learn how to socialize in real life because it deals with attraction and how you present yourself. It's a pretty shady place if you are getting into it to pick up girls, but it is a fantastic resource for those who are simply looking to socialize better in all aspects - not just mating.
Anyways, this response is getting a little long winded so I'll cut it off here - not sure why I spent so much time helping a random redditor, but take from it what you will. Ill delete this post so it only shows up in your inbox and not on the reddit. Here have an upvote, and goodluck dude! :)