Given Epic FAQs the only crossplay Sony supports is the PC. Mobile and NSW support seems to be limited to the XB.
Anyway, my assertion is Sony required that no purchases through Sony’s store could be used on competing platforms (outside of the PC) and Epic simply used a lockout mechanism because it has no way to discriminate the point of origin for purchases.
For all platforms to coexist on the same Epic account and satisfy the before mentioned requirement by Sony, Epic would need a way to parse purchases in terms of their origin and then have a permission system in place so no items purchase through Sony’s store could be used on select competing platforms.
A lockout mechanism is a lot easier to implement if the current system was never designed to accommodate varying platform specific compatibility demands.
Without knowing the specifics of Sony's ToS, though, it's just an assumption on your part that this "parsing" would be enough to satisfy them. Epic's response doesn't indicate it's just a matter of implementation on their end and Sony haven't said anything to contradict their statements so far. This in itself is not confirmation of Epic having their hands completely tied, but until someone else does it successfully I consider it reasonable to assume it's not possible.