I wonder about the practical effects. man xz:

Threaded decompression hasn't been implemented yet. It will only work on files that contain multiple blocks with size information in block headers. All files compressed in multi-threaded mode meet this condition, but files compressed in single-threaded mode don't even if --block-size=size is used.

I assume in the worst case it will just fallback to use a single thread.

Yes, the man page sounds like that. I can't quickly see an explicit assurance, but on the other hand I can't see why they might prefer some other fallback behavior.

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Not motivated enough to test this further. Someone else could pick this up so.