I find this amusing as Apple were the people I had direct interactions with that
didn't run stuff like fuzzers or sanitizers as a matter of course - at least not in the situations I was involved with.
Though this was ~3 years ago now, and a lot of things have changed, but these tools were very much available and well known then - they aren't new. Though perhaps as they "knew" the project was coming to a close it wasn't a priority either?
It also might have fallen through the gaps due to the Apple internal/team culture - I worked for an external vendor, and we had to work against binary built framework dumps that didn't even allow us to enable things like address sanitizer completely either, and fuzzing difficult as you'd need to trace things through their opaque binary layers before it even reached our code.
Apple did have all our code though, it was very much an asymmetrical relationship, but if they were running such things as a matter of course in CI or similar you'd see that pattern in when they reported issues it caught, and the timings from time-of-bug-caused to time-of-report. It instead suggested any such runs were piecemeal and sporadic at best.
Though, it wouldn't really surprise me if they were being run and finding issues all the time, but they never actually got back to us. This certainly wouldn't be the first time we ran into "difficulties" due to the nature of the relationship and culture.