The line about the Soviets confirming it from three continents is the part that got me. Did your uncle ever say what the tracking stations actually picked up themselves, like did he hear the signal with his own ears or just see it on a screen? That difference matters a lot when you're trying to figure out who to trust.

Figured out the same thing the hard way on an old colonial job a couple winters back. Poly bristles just skate across that glazed clay tile surface, especially when the soot has baked on there for decades. Natural bristle has more grab and the tips fray out so they get into the little pits and cracks the poly just glides over. Your brush probably was not worn out, that is just how they act on tile. I keep both in the truck now, poly for metal liners where it works fine, natural for clay. Costs me a few extra bucks a year but saves me an hour of cussing on every old house call.