Just want to add apart from all the other comments. SVT was brought by Intel and spent years to make it extremely well tuned for x86. While not the same for ARM and not even for Apple.

Another point is that the multicore part uses all core including E-Core. On AMD the multicore are all the same. Meaning for some benchmarks this will flavour Apple more.

Again there is nothing that stop people from optimising it for ARM Mac. The problem is the usage of it is so small it probably doesn't make sense to focus on it. SVT took a really long time for it to reach quality parity with AOM's AV1 encoder and later exceed it.