(Answered) macos monterey or earlier photo library import into Sonoma +
(Resolved)
Using the option-key startup of Photos in Sonoma won't allow selecting an older iPhoto library for import. To get around the issue, choose "other". In Finder, find the older library, right-click, select Show Package Contents. Drag the Masters folder from there to the selection dialog in Photos. That should show all of the photos from the library. Review for import, and done.
Posting here because none of the previous questions were answered with this method, which doesn't require running an Monterey installation to do the import.