I know, that's why the foam is the perfered choice for speaker cabinets.
We're talking about 2 different things. I'm talking about using Dynamat in place of cab bracing. what hard, physical bracing does is to stop a vibrating panel. Acoustic foam such as poly batting, mattress foam, fiberglass insulation does not address that, but rather changes the acoustic properties of the cab behind the speaker. Many cabs can benefit from both dynamat AND foam. But they do 2 very different things, acoustically. If you need one, then you very well might need the other as well. And there's no guarantee the Dynamat will solve a boom issue either; bracing may well be needed after all.
And by the way - polyester fill is not really a cheap alternative; IME it's a very effective product used to increase the cab volume that the back of a speaker sees. I've used all of it, poly batting, mattress foam, fiberglass insulation, and personally poly batting would be my
first choice. The fiberglass insulation, while effective, requires using gloves and such to keep it off you skin. DO NOT try lining a cab with fiberglass in your living room as it will get all that nasty stuff everywhere. Mattress-type foam is the worst, as it will eventually break down, especially if stored in heat.
It would not be unusual to open a brand new cab of some high end maker and see something in there that resembles poly batting. But in the end, if you need something to slow down the sound waves inside a cab, almost anything soft will work. Just some better than others.
there are tons of articles online about this, and very interesting.