If they ambushed while the party was asleep, they should catch the PCs prone and without armor, however. That's unlikely to be the case while the spell is being cast.
I've had multiple cases where I was running a published adventure that called for an attack during a Long Rest. That obviously gets negated by Tiny Hut.
But the premise is also that the hypothetical new game is "innovative" and strays further from 2014 5E than 2024 5E does. For those who like 2014 5E, it's a confusing premise.
Is that quote from the 2014 or 2024 DMG?
The 2014 rules made the distinction between "scrolls" (which anyone could read) and "spell scrolls" (which have to be on your class's spell list). But then there was only, like, one mention ever of a "scroll" (the scroll of protection). Pretty much every...
I always thought this was a terrible idea from the moment I heard it was going to be super-detailed graphically. It just seems to work against the imagination element of D&D. But I have sympathy for those who were excited about it.
I kinda think if this was going to happen, it already would have. I won't say it never can, but it doesn't seem likely (to me) to start soon because the zeitgeist moment has kind of passed.
I'm curious about all these unique playstyles that will soon be unplayable due to corporate greed. I'd love to hear from some of these endangered players before this valuable game lore is lost for good!
If you want Mystara in 5E, this fan-created guide is very thorough:
https://www.enworld.org/threads/revised-mystara-players-guide-free-setting-guide-written-using-the-wotc-fan-creation-license.690286/