It's a bad joke, there's nowhere that says OKW can't have medium tanks, but it has been stated that USF can't have heavy tanks.
Whether or not it's mentioned in an official statement or not, the fact of the matter is OKW doesn't get medium tanks under normal circumstances. They get light vehicles, then skip straight to heavies. Yet a doctrine provides them with these Medium tanks that they would otherwise lack. You can certainly argue that it's not a particularly viable doctrine, yet it is an example of an obvious category of units that is missing from their faction that is overcome by an unlockable/purchasable doctrine. It's not like every second doctrine provides them with a medium, in the same way that every other Soviet doctrine provides them with a heavy callin of some kind or an elite infantry unit of some kind and is thus expected to be a standard feature of an army.
These vision statements were not and have never been iron clad contracts. "Air support was virtually non-existent" = they get a pretty amazing strafing run+paradrop ability - coincidentally the USF only get one airstrike. Enormous fuel shortages = cheapest teching in the game, fuel salvage, resource diversion, and the ability to field Panthers faster than the non-shortaged Ostheer faction. Even after the game came out, they experimented with significantly altering the "resource shortage" part of OKW and in one patch literally gave the faction full munitions income. If Relic believes the game can be improved, they're not going to go "oh well, sorry, the vision statement says no!" Particularly if some future DLC pack adds Theatre of War type missions set after the Battle of the Bulge where it would be thematically appropriate to add a Pershing. But nobody should get upset if a Pershing somehow found its way back through the same time portal that sucked the Vampir STG44s from Feb '45 into active service during the Bulge.
USF doesnt need heavy Armor. Why can't you see that? I play USF and realize they don't need it. The idea is you get a strong enough early/mid that you don't need to face roll them with a heavy tank. The factions need variety. Use combined arms, P47's, AT guns to win the late.
It's not needed for 1v1s but would go a long way to making USF competitive in team games, particularly the larger ones.