Here is how a DoS works. Someone gets your IP address. Your IP address is like your home address: it points to you. Specifically, it's a description of how internet data can be sent from anywhere in the world to your router at home.
A DoS works by flooding your router with packets so quickly that its CPU can't keep up. It's literally a tiny computer. In the same way that running DotA on max settings causes certain computers to choke, an incoming flood of packets causes low-power routers to choke.
But remember: When a router "chokes", it can't process packets fast enough. That means you should have observed other internet problems. The fact that you were able to use Mumble continuously means this probably wasn't a DoS.
Of course, Mumble is UDP, whereas DotA is TCP. They're different protocols, and UDP is generally simpler for routers to process than TCP. Is that why Mumble could stay connected while DotA drops out?
No. The only way that would be true is if you had ports forwarded on your router, along with a service running on your computer which was actively listening on that port, like a webserver. In that situation, someone could DoS you by flooding you with half-open TCP requests until your network resources are exhausted. That's a very technical description, but I promise I'm not trying to handwave anything: it's simply how it works. That's how websites like Reddit or dota2.com are DoS'd. And that's distinctly not your situation.
When a player like XBOCT is DoS'd, someone is flooding him with packets so quickly that it overwhelms his router's ability to reject them. That's why all internet services drop, including UDP services like Mumble.
(Sidenote: the fix for this is for players to buy an excellent router. Or to set up one of their home computers as a router. It's very hard to choke out a 2.4Ghz quad-core machine with 16 GB of RAM just by sending it packets! Not impossible, but harder than most script kiddies could reasonably hope to do.)
Now, taking all of this into account, and given that we know Valve's servers are unstable, it seems far more likely that you weren't DoS'd, and that it was just a coincidence.
It might not even have been Valve's servers. Here's how your friends could have experienced problems, while the enemy experiences none: If your friends are located in the same part of the world, but the enemy is located elsewhere, then if a router between you and Valve is having difficulties, you'll get lag while the enemy might get none.
Sorry.