"Does that mean, no matter how many projects I install pytorch and tensoflow and huggingface and all the heavy machinery, they'll be counted only once as long as they're unique?"
I think so, based on my understanding of how this all works. You may end up with different copies for different Python versions, but it should still save you a ton of space.
Just an update for whoever ends up on this comment.
This feature works as long as your venv that uv creates and the uv cache (in user home directory, or anywhere else that is configured to keep the cache folder) are both on the same filesystem.
The drive where the venv was created was NTFS and the user home directory where uv cache exists was ext4 file system.
Therefore the caching didn't work and a warning was shown by uv hinting to the problem.
I think so, based on my understanding of how this all works. You may end up with different copies for different Python versions, but it should still save you a ton of space.