>that assumes that an ARM64 system has some advantage other than address space when compared to ARM32
That is beside the point - the issue is the relative performance of _ARM64_ code when compiled to use different pointer sizes. The advantage relative to ARM32 is irrelevant. ARM64 with 32 bit pointers must outperform ARM64 with 64 bit pointers on important workloads to be worth supporting at all.
There is plenty of evidence to suggest that will indeed be the case, and the difference between LP64 and L64P32 ("x32") on x86-64 will make that even more clear than the current major performance _loss_ one experiences when going from a pure 32 bit to pure 64 bit JVM.
The only way for them to get Websphere on a 64 bit JVM to approach the performance of Websphere on a 32 bit JVM was to use compressed references, i.e. smaller pointers. An L64P32 model on a 64 bit processor is essentially the same idea, and will make essentially the same improvement relative to LP64 without requiring developers to rewrite all the pertinent C code to use compressed pointers by hand.