It's more likely to be something along the lines of skipping a crucial step in adding entropy or subtly botching the rekey cycle so that it leaks session information. If anything like that exists. This may all be a subtle troll of Theo De Raadt, or of the internet at large.

The mail Theo forwards has a vague description of what has been done (not that I understand it, mind you):

"My NDA with the FBI has recently expired, and I wanted to make you aware of the fact that the FBI implemented a number of backdoors and side channel key leaking mechanisms into the OCF, for the express purpose of monitoring the site to site VPN encryption system implemented by EOUSA, the parent organization to the FBI."

Proof? No.

Being able to find a particular commit, made by the people previously accused during the correct time-period, and showing how what they did was subtlety broken? Sure, that's very possible. Proof of mal-intent is hardly necessary.