>What is "Less Secure Apps"? I wonder if my Outlook (part of the MS 365) is such an application.

Nope, it's the POP3 itself, because it allows you to keep your emails, and receive them via who knows which route. With IMAP, the emails live on the server, and generally you can't be sure you can keep a copy, because if it's deleted on the server, it's deleted on your disk too (unless you take the time to create a separate copy).

Anything that's in your hands is less secure than what they keep in their hands.