There's a decent chance that this is also just an issue within a JSON parsing library that someone at R* decided to throw in and use. The parsing library was probably never intended to handle such large blobs, and R* didn't want to mess with the the OSS code.
I wonder if it might even be an incompatible OSS license?
While it's probably not 10%, the transaction cost will come out of the transaction when processed by Epic. Normally that comes out of the 30% that Apple takes. Either way, the purchaser is saving $2 and the developer gets a larger share; this doesn't seem like a bad thing, especially when the developer is going to be spending a decent amount of cash fighting the App Store monopoly.
I wonder if it might even be an incompatible OSS license?