No he couldn't-- the comment betrays a misunderstanding of how Bitcoin and open source development works. "Lead dev" is not a position of authority, and if he wanted to push in a direction the contributors didn't agree with, he'd find himself where he is now, regardless of what happened there...
I don't think opposition to big blocks would be unanimous and the lead dev gets to break a tie-break.
If his decision was bad enough, then the devs who disagreed with him could fork the client.
The miners seem to be taking a policy that they want to stay with core. I think this is to a combination of encouraging stability and confidence in the technical expertise of the core team.
This gives the core team significant power in practice, since a fork has to convince the miners that they should be considered the main development team.
Gavin certainly has less power now than he would have if he was lead dev. If he wants to be able to influence Bitcoin, then giving up that position was a poor move (purely from a strategic point).