If it really did work, I don’t think you would see all those head gasket replacements done.
Edit: I should have noted that a number of materials may help in a head gasket situation, but they are not a proper fix and any real head gasket fix is to replace it while checking to make sure the mating surfaces are flat. In the case of a old marginal car, I might give them a try.
I don’t know which ones are better than others, so if Tester says it is one of the better ones and he uses it, I would consider that a good endorsement.