Is a COM add-in the only way to add an add-in to a Gmail/IMAP account in classic Outlook?

I have an Outlook web add-in (Office.js, MailApp) that works fine on Exchange Online mailboxes. But on a Gmail account added to classic Outlook for Windows via IMAP, the add-in never activates, as I understand it, web add-ins need an Exchange backend to activate against, which an IMAP account doesn't have.

Is that correct? And if so, is a traditional COM add-in (VSTO) the only supported way to surface an add-in on these IMAP/POP mailboxes in classic Outlook, or is there a web-add-in path I'm missing?