Good points, but careful of the logic; yes there's a lot of energy being produced and plenty of harmonics etc. None of which says that every material heats equally well, with no passthrough, or that microwave ovens with different frequencies wouldn't be more efficient for other specific materials.

If a material is dense enough (metal being the prime example) it'll block. I've nowhere argued that you could send microwave power through solid walls - whether brick or sapphire economically.