You’re right @grb.
Actually, I just found a better solution:
The export I originally did from Roboform was via the Windows desktop client.
I now tried importing a file that was exported from the Roboform browser extension (in Firefox, I assume that the other browser extensions should give the same result), and it was successful!
The exported file had different field names, apparently the ones the Bitwarden expects.
So the bottom line is - if you need to transfer data from Roboform to Bitwarden, use the Roboform browser extension to do the export, and not the desktop client.