The OKW lack a medium tank, yet they can get a medium tank through doctrines. It's that simple. OKW are designed to not have a medium tank. But with the right doctrine, that's all upended, because the faction designs ARE NOT RIGID
OKW wasn't designed with "No Medium Tank" as a faction statement, and the OKW Panzer IV despite being rare and currently underpowered, was prominently featured in many pre-release screenshots. Rather, adding an M26 to the US Faction would be more like a "OKW Supply Doctrine" that gives 100% resource income and allows the construction of caches. That goes against the faction design.