I think the metric Epic would be interested in would be the amount of royalties they would collect, so it would include the number of developers scaled by the financial success of their games. The financial success of the developers whose market share you capture matters a lot.

Godot is more likely to be used by people whose games wouldn't earn enough to cross the threshold where they would owe money to Epic had they used UnrealEngine, so keeping them away from Unity makes a lot of sense.

Only if you ignore the network effect of developers knowing your platform vs others...

Somewhat true. While it doesn’t matter to the end user what engine is used if it looks and runs the same, it does if one engine can’t compete in polish. And I don’t know about you, but I hate splash screens that just say (for example) “built with Unity”. (Yes, splash screens hide loading, but as a user, I don’t care what engine you used)