> the mullite bricks certainly are good insulators themselves. As for the concrete ones, they're puffed up with air and also ok.

Sure, with some exotic materials and fancy tricks you can get something possibly approaching mediocre with bricks. This is fairly niche and not part of the general calculation. I'm happy to revise my opinion if you show a type of brick that is readily available and commonly used for building that has a useful R-value. As far as I'm aware, that doesn't exist outside a few niche products.

> I don't get how "reducing heat transfer" is not "insulating" though.

I never said that.

> Another plus of bricks I left out is soundproofing.

Sure. There are many advantages. I only and specifically took issue with your claim that brick is a good insulator, because in the general case it is not.