It's clear that Arm worked with Apple on AArch64 but saying it was basically designed 'by Apple' rather than 'with Apple' is demonstrably unfair to the Arm team who have decades of experience in ISA design.
If Apple didn't need Arm then they would have probably found a way of going it alone.
Apple helped develop Arm originally and was a (very) early user with Newton. Why would they go it alone when they already had a large amount of history and familiarity available?
If Apple didn't need Arm then they would have probably found a way of going it alone.