Floor heating won't go from heating temperatures (30°C? I don't know exactly) to freezing across 8 hours of sleep, even if it's -25°C outside.

The advice also doesn't have to be "turn it off", just "turn it down to 5°C" which would be safe indefinitely. There has to be another reason why the advice is to keep heating 24/7.

The controls available aren't in the sense of thermostats - for example, you can't ask it to keep temperature at 21 degrees during the day and 18 degrees during the night. Instead, it has something called 'heating profiles'. There are summer (passive cooling, essentially) and winter profile (heating). And for the winter profile you can set the level of heating: from -5 to +5 degrees compared to what it considers optimal. I have mine always set to -5, because even at that setting it's warm enough. Note that 'degrees' in this instance is not the air temperature in the rooms, but I believe the water temperature in the pipes.

In other words, it will automatically adjust depending on the time of day, temperature outside, etc. It is possible to influence how it works, via 'administrator' settings meant for the repair guys, but I haven't touched those.

Well that just sounds like a badly designed heating system then.

I have radiant floor heating, and setting the thermostat to my target temperature 3 hours before wake-up is fine.

But note that the OP has not just radiant floors, but also a heat pump (which isn’t using gas, so we’re far afield from the overall question, but setting that aside).

Heat pump systems are generally sized so that compressor’s sweet spot is pretty close to your expected normal/max output.

To be honest, this is more about the lifespan of the compressor than it is energy efficiency, but there is also a small loss of efficiency if you get the system into a state where it’s turning on/off frequently.

So it seems plausible and sensible that a radiant-heat pump system would recommend circulating 24/7 in the winter rather than cycling on/off a lot.

I don't know. It's not what I was used to before in my previous homes (thermostats in different rooms, specific temperature settings, etc.), but you know what? It keeps me warm and is cheapest heating I've ever had to pay for, by far.