It's worth noting that TF2 also has a default client update rate of 20/s, but one of the first things you should do (if you have a steady connection on something that isn't dial-up) is change it, especially if you want to play sniper.
That the servers run at 60/s is far better to know (it allows more accurate differentiation between actions that would otherwise be considered simultaneous, and limits the minimum interpolation interval and client input rate). If it were 20/s, and every action was lumped into 50ms spaced updates, you can imagine how horrible that would be. It also sounded like from that video in the link that client inputs are also sent at 60/s, which is really the most important, so it's just the clientside interpolation that's crap. The difference between 50ms and 15ms will often come into play if you're trying to get a very precise hit on someone who's trying to evade it, like when going for a headshot with a hitscan weapon.
That they're asking for people to run it on custom servers first is amusing, as is that "20% of players wouldn't be able to play" figure. It should just be an option, like everything else. Competitive players won't take 20/s interpolation seriously, but if your connection can't handle it, the majority of people won't notice the difference, except when using perfectly accurate hitscan weapons for precise shots.