This is not authoritative but as far as I can remember, a depot file is actually not a *real* depot but a collection of files, imitating to be on a tape. And since sw commands do not support remote tape drive access, what you are seeing makes perfect sense.
What you need to do is to copy the contents of this depot file into a directory, say /tmp/mypatches, using swcopy command, then register this directory as a depot with
swreg -l depot /tmp/mypatches
then you should be able to access it from remote systems.
HTH
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UNIX because I majored in cryptology...