Don’t be surprised in case you find out your view on this matter might be the outlier if you check back with the internet.
Edit: I just checked, and my Fairphone 3 behaves exactly the same as the Fairphone 4 mentioned in the linked topic.
If powered down, it will power itself up 2 minutes before it can then sound the scheduled alarm (although it just vibrates and doesn’t play the alarm sound, might be a bug).
So, I would count this as expected behaviour.
Keeping the power button pressed for about 15 seconds should force a reboot in any situation.
Smartphones have the Do Not Disturb mode for what you want to achieve, you can enable it at will in the quick settings menu (the one you get by swiping down from the top) or in the Settings for a set period of time or for the time until you disable it again. The only thing left to do for you would be to check in its settings that it also silences alarms for you:
Settings - Notifications - Do Not Disturb - Alarms and other interruptions