An alarm is something urgent, 3 days and 5 hours from now isn’t urgent, that’s something you need to be reminded about, which is what the reminder prompt is for.
An alarm is something that warns or signals, by one definition. It doesn’t necessarily need to be urgent.
However, Google is weird. At least on my phone, I can easily set a timer for 3 days and 5 hours without a problem.
Alarms on Google typically mean wakeup alarms. That may trigger the snooze/cancel functionality when that timer expires. Odd that you can’t set one way ahead of time.
I am not trying to be a jackass here or anything, but I have never actually thought about the distinctions between alarms, timers and reminders. I just kinda thought they were all the same. /shrug