Can thief unlock stolen phone that was locked by Android Device Manager? My phone was recently stolen. 

Luckily, I enabled Android Device Manager awhile ago. So, I locked the phone via the web app of Android Device Manager.

Can the thief unlock my phone? I'm not asking how. There is a scenario in my mind that I'm afraid that could be possible to unlock the phone and restore factory settings, wiping all my data. Is that possible? 

Again, I just want a Yes or No answer, not how he can do it.
The answer is almost certainly yes. 

Someone could probably get past it in one way or another. (They could probably just use odin on a samsung device for example. Though I have never done something like that myself.) If you have sensitive data on there then a remote wipe might be the best thing.

Did you report it stolen with your carrier? 
They could report the ESN / IMEI  as stolen so at the very least the thief probably wouldn't be able to sell it.

Of course I do not know your exact situation. If you know for sure it was stolen and not lost could the police recover it from the thief?

Either way best of luck to you.