I would say they are wrong. On my 1997 +4 there was an L shaped steel lip on the frame. I hope you can see that on the picture which I had to blow up to show it.
[Linked Image]
Hopefully an owner of your yeays car will confirm you design. Try BHM spares as they can I am sure help.

JohnV6
2022 CX Plus Four
2025 MG ZS EV aka Trigger