Yep, antimatter can be contained... the problem is containing very much of it.

At least for now, miniscule amounts at best.
As for shooting down multi-warhead ICBMs being better because you save lots of places at once... that would require being able to hit them before they split apart, which they do at the apex of their flight. Which is IIRC pretty much above the atmosphere, and out of the range of just about anything you can think of.

But if you could shoot them on their UPWARD track (not bloody likely) then it would work.
But anyway, back on topic... no need to make the Meson Bomb defend itself. I wouldn't mind if it was a bit bigger, though...
