It appears they've backed off on the royalties, too: originally, it was 5% of gross[1], and now it's 5% of gross after $3000 sales per quarter.[2] 1 https://www.unrealengine.com/blog/welcome-to-unreal-engine-4 2 https://www.unrealengine.com/blog/ue4-is-free
Other, much lesser known, engines have attempted to do something of this sort by acting as an intermediary eg. publisher, planning to pay developers after they were paid by Apple. For obvious reasons that was immediately dismembered by the community.
The only way I can figure is they'll ask for an iTunes connect login so they can check on the numbers for themselves.
Speaking as a developer, it just sounds like something to be avoided from the start.