I don’t see GM fiat in there anywhere. I see your chance of success on an action if it is possible is an ability check or a saving those. And for some things a flat random chance that is X d6. Just like any other rpg when something comes up. Make a dex check. Make a strength check.Yeah. But without proper guidance of what the RAI is, the RAW will win out. And we've established that the RAW isn't the "true" way to play a game in the OSR mindset. The RAI exists only in the memory of Gygax and Arneson, creators like Rob Kuntz who post on here, etc. The books should reflect how the game should be played, not with an asterisk that means "yeah, but we don't really play it that way." If we're going to be preserving this playstyle beyond just some sort of oral tradition it needs to be carried on in print, and the OSR is best situated to carry on that legacy (rather than just reprinting and slighting reformatting the games from 40-50 years ago).
You're right. But that's how it comes across in play regardless. Unless you have access to that RAI and decades of evolution and houserules, the game plays exactly that way. Especially for new players - who, if we're being honest, probably came from more modern rules that are more explicitly spelled out and will look for "official rules" other than GM fiat.
The issue is that the books are inconsistent, and if an inexperienced gamer comes across something presented as a rule, that's going to be what they go with. Granted, I am not well versed in every OSR product, I can point out examples from OSE, since it's what I have played most recently, have readily available, and is sort of the industry leader ATM.
"Hit Points (hp) The character’s ability to avoid dying. The character has a maximum hit point total and a current hit point total, which are tracked separately. When a character is harmed, their current hit point total is reduced. If this number reaches 0, the character is dead! Rest or healing can restore lost hit points (see p104), but never above the character’s maximum hit point total (this is only increased when the character increases in level)."
"7. Roll Hit Points Determine your character’s hit points by rolling the die type appropriate to the chosen class. Modifiers for high or low Constitution apply (see Ability Scores, p16). Your character always starts with at least 1 hit point, regardless of CON modifier. Re-Rolling 1s and 2s (Optional Rule) If your roll for hit points comes up 1 or 2 (before applying any CON modifier), the referee may allow you to re-roll. This is in order to increase the survivability of 1st level PCs."
"Death: A character or monster reduced to 0 hit points or less is killed."
You don't get death saves, no dying out. You're just dead. End of character. OSE at least as the courtesy to let you re-roll 1s or 2s at 1st level as an optional rule.
"The referee may use a character’s ability scores to determine the character’s chance of succeeding at various challenging tasks."
OSE p216 [bolded part mine]
"Resolving actions: When a player wishes to do something not covered by a standard rule, the referee must consider how to determine the outcome. Sometimes, the situation can be dealt with simply by deciding what would happen. Sometimes, the referee may require the player to make an ability check (see p104) or a saving throw (see p105) to determine what happens. Other times, the referee may judge the likelihood of the action succeeding (e.g. expressed as a percentage or X-in-6 chance), tell the player the chances, and let them decide whether to take the risk or not."
So there is a section, in the chapter for how to GM the game, a single line that vaguely says "GM fiat." Otherwise, there are twice as many lines in the same paragraph that talk about the rules - not to mention the previous chapters (also geared to players) about character creation and the rules system itself that take great pains to describe the method of mechanical resolution.