I’ve done exactly that and have witnessed others doing exactly that as well. If aliases were unique, meaning once you registered an alias NO OTHER PERSON COULD USE THE SAME ALIAS, then having an alias would make you somewhat identifiable. You still wouldn’t be identifiable if you were to play on a different account with a different alias. However, anyone can have any name they want, and at times there are dozens of people with the same name. Barcoding is a great example of this in action - dozens of people with the same name that are hard to distinguish because they have the same name. Ergo, knowing someones alias doesn’t tell you a blasted thing since it could be anyone.
The bottom line is that you are playing against someone who by definition is anonymous - they are some random dude somewhere on the planet, with a random name that he picked, which name can be anyone elses names, and he can change his name at will to any other name. SC2 is an anonymous sport. That’s how it is. Calling someone a coward based on their anonymity is illogical in an anonymous sport.