Yes, precisely. Subdir was added after that variable, and was the same each time - the only dynamic part was handled by the symlink. So I suspect, as @uok said, restic just saw it as the same directory structure.
I didn’t benchmark my run at the time, so dangerous to comment too confidently on performance. After the first snapshot was imported, I remember subsequent ones were definitely faster. Don’t know if that was purely because deduping was avoiding things being written (which in itself is a nice speedup), or if there was more intelligent scanning of the source. I was probably also being impatient (a watched progressbar never completes). On balance, I would trust @uok’s current experience a lot more than my own recollection 
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