I don't want to sound like a 5e apologist here, but I'm pretty sure I'm going to, anyway. Apologies (npi/maybe some irony though) in advance.
I've played in an RPG where the players knew no rules and the DM, [MENTION=813]jmucchiello[/MENTION] , adjudicated everything. It was a lot of fun, but there was trust and consistency from him.
The 1e DMG advised DMs to keep ahead of their players in rules-knowledge. Your anecdote is sort of an extreme/ideal case of that with players having 0 rules knowledge, but it does illustrate that that the players having no recourse to rules-'lawyering' can work very well, as long as the players trust the DM.
In an RPG with written rules, that's a common foundation for everyone playing the game, with the DM both as arbiter and custodian of the rules. If there is ambiguity, like in this case, it's up to the DM to decided how to run it. And if there is no ambiguity but the DM wants to run it otherwise, they can do that as well as a house rule.
That's a fair way to handle it, but not the only way. And, I'd argue, not quite the way 5e was designed for, ideally. You certainly can run 5e with above-board formal house rules. You might find you need a lot of 'em. IMHO, running 5e on "rulings not rules" works even better. You do lose the implied consistency of formally changing the rules, but you're not boxed in by those changes, either.
However, it's the shared understanding of the rules that allows things to flow smoothly.
That's another way. You can depend on the DM, or depend on the rules, or both to varying degrees. Depending entirely on the DM (Empowering the DM as 5e does), or on the other extreme, sticking to a clear/balanced/consistent ruleset, can help a game run smoothly. In both cases you avoid having too much debate or animosity over rules issues.
Also, if the DM runs a rule one way in one circumstance and another way at a different time, the players need to bring that up as well.
It depends on the group's style and the game in question. It may well be that there is a good reason for the DM ruling differently at different times. (Of course, there could be a 'bad reason,' that's why the afore-mentioned trust is so important.)
Not saying a DM can't change how they handle something, but it should be like the OP - the DM comes out and says "I'm going to be doing X differently because of Y and Z". Because at the end of the day RPGs are a social game and everyone has a say.
The implied social contract can vary. You can't force a player to join or stay in a campaign, for instance, so everyone has a say in some sense, no matter how tightly the DM holds to his prerogatives.
5e's loose design and DM Empowerment help it work very well when the players trust the DM to handle things with rulings and don't dwell on whether they're consistent or not. That's further facilitated by the old-school technique of taking a lot of the resolution, bookkeeping and the like 'behind the screen,' removing sources of doubt and/or contention.
Not that there's no merit in discussing what might be a better ruling, just that it's a DM discussion. There's no need to 'prove' or even argue that RAW goes one way or another, for instance.