I'm not sure that is accurate for Gravaton3, which appears to be besting intel/amd in a number of cases. So, it doesn't appear there is a clear leader between the amd, intel and gravaton instances. Meaning everyone should be benchmarking their application and picking one.

https://www.phoronix.com/scan.php?page=article&item=graviton...

Amazon here seems to be leading the Arm technology pack, as they are using a newer generation of CPU, while arm instances on other providers tend to be providing gravaton2 (Neoverse-N1) based instances. I would imagine that gradually changing in the near future as those vendors also upgrade.