I have never heard about this before, so I think this is a new rule for 2020. Correct me if I'm wrong anyone.
Off course there are a set of rules for what is allowed, but inside those rules the manufacturer can do what they want.

This reminds me of RC cars where you buy the base and then you buy different body shells and change as you wish.

I guess it evens out the competition and the better drivers will stand out. But with engines coming from different suppliers, it won't just be up to the pilots.