Boris is doing what Boris does, he's airing his views for personal reasons. He doesn't seem to mind whose toes he treads on, and he soaks up Theresa May's irritation like a sponge.
He hasn't really been tested in his role of Foreign Secretary yet, and that will be interesting when/if it happens. I think I've said this before - Boris is not the buffoon that a lot of people think. Underneath all that waffle and blunder ticks a very astute mind.