Sounds like their car washing equipment probably damaged the windshield and they are attempting to blame you.
It is certainly possible there was a small chip in there that grew exponentially a few hours after you returned the vehicle... it is also possible pigs can fly.
This is a bogus claim.
Any idea if there were cameras in the return area recording the return transaction? I'd like to have them pull that tape to prove it was cracked when you returned it.
Given how many staff they've laid off you'd think the people who are left would be ethical and honest and glad to not have been laid off. Very sad.
Also those "close up photos" you were provided- is there anything on those photos that even proves this was the vehicle you rented?