After writing this I decided to look at my shortcut. The action seems to have been a simple "get directions to <place>" and sent verbatim to Siri.

I was not able to edit / update it! However, there was now a new "maps" option for `Open <type> directions from <Start> to <Destination>`

Where type can now be {driving,walking,biking,transit} and <start> is Current Location by default.

After updating, this now seems to correctly set actual driving directions, even if I'd previously set up a walking route!