LOL
Well, there's a bit of truth with the Bluesbreaker cloth; back when Marshall first reissued the BB (a combo amp), the cloth they used for
the cab was incorrect (along with many other things including the actual circuit). Apparently Marshall ran out of the real stuff long ago.
Since then, those who were fortunate enough (or had the hindsight) to have rolls of this particular original cloth have been making a mint.
The other piece of truth is that it does effect the tone; some of the Marshall cloth ("basketweave" for ex) over the speakers was very heavy and thus diffused the highs to a degree.
It's not much different really than taking a "naked" speaker and draping a towel over it and playing, although this would be exaggerated no doubt with a thick towel.
But as far as "holy grail" tone goes, definite BS.