You can definitely have pages that Google considers "mobile-friendly" without using AMP. Do you have a viewport meta tag on the page? If you have any CSS at all some mobile browsers will default to a desktop view that requires zooming to read, and that's probably what Google is detecting.

Regarding the analyzer tool, I suspect this is a case of Google's left hand not knowing what the right hand is doing. As I recall, Google is in the process of switching from a static analyzer to a Chrome Lighthouse-based analyzer (or something to that effect) and I'd guess that the mobile-friendly rules are slightly different between the two.