I wouldn't be surprised. I mean, Johnson's a cunning narcissist who wants to be remembered as some great prime ministerial hero alongside his idol Winston Churchill. He himself backed Truss for PM and he must've known she would flounder spectacularly.
For this contest, there's a 100 MP nominations threshold for hopeful leaders and Johnson already has 50. As an aside, Truss only got 88 during her bid, so wouldn't even qualify under these new rules.
The depressing fact is that, despite Johnson being horrid, the Tory's don't really have anyone better to replace him as leader. At least from the perspective of being able to win an election*. That's why it took so long for them to oust him in the first place.
*As evidenced by a BBC vox pox last night who said they think Johnson should be the next leader because he's a "good character".