It's as if they had to do some big R&D project to figure out how to sync separate digital audio receivers. We've been doing things like this in the open-source world for years now--I hacked together a synchronized multi-sink audio system as a weekend project in 2011, for example.
That was UDP/IP/ethernet, but the same principles of latency-matching apply pretty much regardless of the underlying transport.
I suspect that this actually has nothing to do with "[having difficulty figuring out how to] ensure that both earpieces receive audio at the same time to avoid distortion"--because, frankly, I don't think the people at Apple are so stupid as to think they need to invent latency-matching themselves and then also have difficulty with it.
They're probably just having difficulty sourcing parts from one of their vendors or something, but claims like `oh it turned out to be very hard to synchronize playback wirelessly because we're breaking so much new ground with multiple independent receivers' reads as much more profound to the general public who don't actually even know where to begin thinking through something like that.
Having the universe conspire against you with things like physics and math, vs. having let some plebeian manufacturing house upset your schedule due to retooling-problems or materials shortage or some local holiday you didn't know about... or whatever. Which would you rather have as part of your narrative?