Maxperson
Morkus from Orkus
No, it's not. Obviously the DM at a table can rule that it is, and he can DM for an empty table, but that is not an argument that would find much traction in the overall popular culture of D&D. Fluff text is just that, fluff. It provides an example of what a typical example of someone who is part of the class would be like. The idea that a high priest can demand your cleric to obey... I'm honestly not even sure if you're serious. If you are, two responses:
So you're fine with the fluff of a medusa's gaze being, "The gaze of the medusa causes tickles and laughter"? You're okay with the fluff of a castle wall being, "this tall oak tree stands 150 feet high and is full of squirrels"? Fluff dictates many of the mechanics and is ironclad if it isn't changed. Fluff more often dictates other fluff and how different pieces of fluff interact with each other. This latter type is what I'm talking about with warlocks. With the warlock class, you are sworn and beholden to your patron, because that's what the fluff says you are. There is only one way to alter that, and that's for the player and DM to agree to different fluff.
1. Sure. The high priest can demand service from your cleric because the fluff says so. It doesn't say the cleric has to obey or that ignoring this fluff will have any mechanical impact. Hell, anyone can demand service from the cleric, and he can subsequently ignore them.
The high priest is called out specifically, because to a cleric, he has the authority and position of the president of the company and you are just a junior executive. Sure, you can refuse him, and there will be good roleplaying and consequences to such a decision. Or you can obey, and there will be good roleplaying and consequences to obeying. Unlike if the cleric ignores the demands of farmer Joe.
2. You suggesting that the outline of what would typically be true of an archetype, such as warlocks having to provide periodic services to their patron, is as ironclad a rule is just silly. It's also immediately contradicted by the fact that it also states, as flavor, that if your patron is a Great Old One they might not even know you're there.
It's ironclad that you are obligated to perform tasks if requested. And you're right, should the patron be a Great Old One, I would probably never make a demand. Or if I did, it might be something that seems really irrational like moving the horse trough in front of the inn across the street. That doesn't change the fact that the fluff is ironclad unless changed by the DM.