Yes I do understand how modeling works.
I've also used metahuman.
Metahuman just gives you an FBX. You CANNOT edit it at runtime like you can with UMA (which is why the player model is UMA, because you can morph the bones, swap clothes in and out).
Therefore, no they don't share more in common than their total differences.
The closest in the unreal store to UMA is this:
https://www.unrealengine.com/marketplace/en-US/product/universal-character-customization-system?sessionInvalidated=true
And 2 points you're obviously missing.
1) UMA IS ALREADY IN THE GAME!!!! The player uses it.
2) Updating that and making clothes, models, skin maps, hair, etc... would take less time than the "in house" system they apparently want to do. Simple as that.