[syncsynchalt]

Bitcode allows you to optimize between chip versions with different opcodes, but not different architectures, word sizes, or endianness.

Source: the recent Accidental Tech Podcast that interviewed Chris Lattner.

[abalone]

Ok fine, but the broader App Thinning featureset of the App Store still means they wouldn't do a universal binary. They'd deliver just what the target device needs.

[gpderetta]

True, but AArch64 vs AMD64 are not so different (64 bits, little endian, probably quite similar ABIs), it is not unthinkable that the same bitcode could target both architectures.