There's no excuse really.. No matter if you have good programmers or bad programmers, you should always test that your software works before issuing a new version. If the customers have no choice in whether to upgrade or not, and they can not choose to downgrade if there's something wrong, then testing scheme needs to be very strict. Stuff like this should just not manage to get into production.
If the problem appeared 10 days after patch, and only for some users, then one could always argue that it is hard to test all cases, so it can happen, but then it happens to everyone immediately, then they do not seem to have tried to test that multiplayer works with patch at all.
Anyhow.. I guess the root of it might be that when steam makes updates, it doesn't test that every game still works before disting it. Appearantly not Civ V at least.