The Intensity Minutes feature clearly only includes tracked activities and ignores non-tracked (Move IQ) events – although it's supposed count both:
https://support.garmin.com/en-US/?faq=pNU9nnDzzGAHmEavp9rpY8
I've walked 10000+ steps every day (bar one) this week – including long, brisk, continuous walks – but Garmin Connect only counted 5 Intensity Minutes for the whole week. So today I manually started a "Walk" activity, did a regular, leisurely walk and voilà, almost every minute of the activity counted as an intensity minute (although I even stopped a few times). That's not how this feature is supposed to work.