Recompressing should be guaranteed deterministic. It’s the packing/unpacking of tar archives to/from directories on disk that leads to the non-determinism (such as timestamps and ownership metadata). If the tar is left intact, both zstd and gzip should produce byte for byte identical outputs given the same compression parameters.

You are correct; I confused archiving with compression. However, even considering only the compression process, same compression parameters cannot be guaranteed, as it is unknown which compression parameters the image publisher used.