Is this proof he knew bridges were closed or that he knew they were closed for spiteful reasons? The former is NBD. the latter is obviously problematic.
If he didn't have that press conference even if he ordered the closing that wouldn't have been the dagger..It would have hurt, but it wouldn't have been a killer.. What's going to destroy him is that press conference where he swore up and down he didn't know.. If he did know, he's done..
Nixon knew nothing about the Watergate break-in.. But it was his lies later on and the cover-up that sunk him..