Looks like a lot of unrelated error are hidden behind "unknown SWHID". For instance, using the Web API via the VPN with the VPN down, results in that error too, with the syscall hanging.
There seems to be two things to fix about this:

• more appropriate error reporting (rate limit exceeded, can't connect to API server, etc.)
• the syscall should not hang, but rather return the user space an appropriate failure (note indeed that these hangs are bad, of the kind you can't Ctrl-C out of)

In a different test, I've seen the Too many request error, but only after Ctrl-C, with an asyncio issue:

ERROR:root:Unknown name during lookup: 'HEAD'
   unique: 1866, error: -2 (No such file or directory), outsize: 16
^CERROR:asyncio:Task exception was never retrieved
future: <Task finished name='Task-45' coro=<_session_loop() done, defined at /home/zack/.virtualenvs/swh/lib/python3.8/site-packages/_pyfuse3.py:28> exception=HTTPError('429 Client Error: Too Many Requests for url: https://archive.softwareheritage.org/api/1/content/sha1_git:642be81e2679d4524e736bafe6c4576360a9fd5c/raw/')>

This might be the key issue here, a missing (async) exception handler somewhere.