However, I note that "export to pdf" is grayed out for a file that is already a PDF ... which I don't remember being the case ...

Just… use the existing file.

You receive a non-editable pdf ... you use the textbox and signature tools in Preview.app to mark it up and sign it ... and then you export it as a pdf to send back.

How else would I complete that workflow ? Genuinely interested ...

Preview lets you save any annotations to PDFs and images it supports. It’s literally as easy as pressing Cmd-s. If you really want to save it as a different file, use Preview’s duplicate feature to save it as a new file.

If you want to make the file you’re editing non-editable/flattened, then you can use macOS’s built in print dialog which lets you natively save to PDF. This isn’t a Preview feature per-we, but it is system wide.

This seems to have not been possible before and changed in Big Sur: https://discussions.apple.com/thread/252703055

And Apple’s guide considers Save and Export to PDF to be the same function: https://support.apple.com/guide/preview/annotate-a-pdf-prvw1...