See... I get this in an absolute sense, but not in a practical one. There's some wiggle room here, that lies somewhere betweeen "solid walls block AMF from extending to the other side of the wall", and "squishy/softt/thin things block the AMF as well".
So... If I hold up a sheet between me and Sunny, can I block the AMF and cast spells?
What if I have a ring of regeneration on my finger and am wearing a glove on that hand? Does clothing block AMF? So... I wear a beekeeper suit and I"m golden?
One either has to rule that "a person and all objects on/in their person" is in the AMF, or... not. I would tend to rule that if Caulder's body is in the AMF, then anything inside his body is also in the AMF. And yes, that would include someone/thing that he has swallowed. Ruling otherwise runs you down a rabbit hole of increasingly strange rulings.
I'd say that solid walls/floors block AMF, but nothing else (maybe a cabinet would work maybe?). I would certainly *not* rule that living beings bodies block AMF at all. Again, because otherwise, you get into silly territory with the effect.