Donny could have won the game. Reveal TA, make a risk, give up that 20k, flip the whole house on Frankie and Derrick, form some other majority alliance, won. Jen Johnson gave up 250k (potentially) to save herself when she was getting evicted for sure. That was one of the best moves in BB. Couldn't you all see Caleb screaming incoherently about "I WAS BETRAYYYYYEEEDDD" and he would have convinced everyone just as easily as he did that Zach was the saboteur using the same tactic. Instead Donny choose to lose. SO there's that.
No1 would believe him because Derrick and Frankie would deny it. It's going to be his word against theirs. So he would not only lose the money, but still get evicted.