It's pretty simple really. He's confusing "divine" with "infernal" or taking a really holistic definition of divine to include infernal because why should the actual English definition of "divine" meaning "sacred" be taken to not mean "of hell or sacrilegious"?
I can see his point because popular media has made the point of questioning the lens that we look at good through since at least the 60s, (one popular example being Return of the Sith, Anakin's "I think the Jedi are Evil" approach).
That said, the DM's world view of what English vocabulary means in the context of the rules is a social issue, not a rules issue. My own point of view is that English vocabulary is interpreted the way it's supposed to be, not the way media colors it.
KB
While what you are saying is true about the English language, under Warlock it specifically states that Patrons are not gods. Since they are not Gods, they cant grant divine powers, which is what fuels Paladin powers. From the PHB:
"A paladin swears to uphold justice and righteousness, to stand with the good things of the world against the encroaching darkness, and to hunt the forces of evil wherever they lurk. Different paladins focus on various aspects of the cause of righteousness, but all are bound by the oaths that grant them power to do their sacred work. Although many paladins are devoted to gods of good, a paladin’s power com es as much from a commitment to justice itself as it does from a god."
" The most important aspect of a paladin character is the nature of his or her holy quest. Although the class features related to your oath don't appear until you reach 3rd level, plan ahead for that choice by reading the oath descriptions at the end of the class. Are you a devoted servant of good, loyal to the gods of justice and honor, a holy knight in shining armor venturing forth to smite evil? Are you a glorious champion of the light, cherishing everything beautiful that stands against the shadow, a knight whose oath descends from traditions older than many of the gods? Or are you an embittered loner sworn to take vengeance on those who have done great evil, sent as an angel of death by the gods or driven by your need for revenge? Appendix B lists many deities worshiped by paladins throughout the multiverse, such
as Torm, Tyr, Heironeous, Paladine, Kiri-Jolith, Dol Arrah, the Silver Flame, Bahamut, Athena, Re-Horakhty, and Heimdall. How did you experience your call to serve as a paladin? Did you hear a whisper from an unseen god or angel while you were at prayer? Did another paladin sense the potential within you and decide to train you as a squire? Or did some terrible event—the destruction of your home, perhaps— drive you to your quests? Perhaps you stumbled into a sacred grove or a hidden elven enclave and found yourself called to protect all such refuges of goodness and beauty. Or you might have known from your earliest memories that the paladin’s life was your calling, almost as if you had been sent into the world with that purpose stamped on your soul. As guardians against the forces of wickedness, paladins are rarely of any evil alignment. Most of them walk the paths of charity and justice. Consider how your alignment colors the way you pursue your holy quest and the manner in which you conduct yourself before gods and mortals. Your oath and alignment might be in harmony, or your oath might represent standards of behavior that you have not yet attained."
I see "god" and "gods" and "holy" and "sacred" in there many times. Also there is a clearly stated "
Different paladins focus on various aspects of the cause of righteousness, but all are bound by the oaths that grant them power to do their sacred work."
As far as the instant example, of course it will be stated "that's just fluff and not an actual rule so its meaningless I will recreate my own fluff that is meaningful and overrides anything the DM says about his world and his gods and that's what I will use."
