Continuous monitoring is only claimed during a recorded workout https://support.apple.com/en-us/HT204666

That page doesn't say specifically, but my recollection is that it's otherwise every 10 minutes if you're still. In non-workout mode they don't measure heart rate if you're moving; presumably not accurate enough.

All-day heart-rate monitoring can only be claimed by devices with a battery life of at least a day.

Neither page offered any sort of metric on what constitutes 'accurate'. so how do you know FitBit doesn't just have a lower standard than Apple does?

right... I mean "continuous" on a digital device is still going to have some sample rate... Apple Watch has a 10 minute sample rate most of the time and some higher sample rate when they are doing "continuous" monitoring during a workout. Fitbit must have a great way to store that data if they really are doing "continuous" monitoring all day.