Mine didn't have HR monitoring, but I found the movement (aka actigraphy) to be pretty much useless. Fitbit was counting all the time I was laying still trying to fall asleep as "deep sleep", and looking at the chart I couldn't differentiate movement at 12:00 when I knew I was awake from movement at 6:00 when a noise woke me up but I fell right back asleep.
The movement data is probably enough to tell when I've slept better or worse, but I wouldn't trust the absolute "number of hours of sleep" they report.
Fitbit does not attempt to track sleep stages w/o HR monitoring; it's just Awake, Restless or Asleep. Yes with nothing but accelerometer data stillness can be mistaken for Asleep, but I think you're misremembering when you say "deep sleep".
The movement data is probably enough to tell when I've slept better or worse, but I wouldn't trust the absolute "number of hours of sleep" they report.