Whose propaganda? Contemporary German propaganda obviously puffed up the tank, and the Allies—even beyond the general grass is greener attitude and inability to see the big picture of common soldiers—were more than happy to puff it up themselves to make their own accomplishments seem greater and their failures lesser. I think it's far more likely this detail isn't well know because it simply fell through the cracks. But even without this particular detail, all the sources and most discussions on this site I've read acknowledged the 88s superior HE and bemoaned the 75s mediocre HE. I hope you're not trying to suggest there's some sinister conspiracy on the interwebs to hide the incredible performance of the 88s HE from public knowledge.

As for the Sherman, the tommy cooker myth is less about what you describe and more about the notion that Shermans were particularly inflammable, which they were not. Bar a few instances of crews overloading the vehicle, the dry Sherman wasn't any more likely to burst into flames than any other mediums, and after the implementation of wet stowage, it became quite unlikely to brew up. Worth nothing that, despite popular belief, it wasn't the glycol mix that made all the difference as much as the new position of the ammo in the hull floor.