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:
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.
2. The fluff descriptions are not instructions on how to play or rules, they are flavor. 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.
Fluff is fluff, it's not rules. Suggesting there is a parity between changing the effect of spells and the damage dice of weapons and tweaking class flavor a bit is just.. it's ridiculous, and it's very much against the spirit of Fifth Edition.