So i just logged in after an hour and got this penance incl. the steam achievement. I think the following happened:
My last game started and i fought a horde with bots. Someone joined the game, which caused my game to crash, but i blocked at the same moment. So i guess some form of “ghosting” happened on the server where my character still stood there with infinite blocking for a couple of seconds.
Nothing special, but it shouldn´t be a thing to still be able to cheese penances that way. (Which could be done with closing the game i guess. Worked on others in the past aswell.)