The description mentions WLK as an very cleric like class but warlocks originated on 3.5e Complete Arcane sourcebook. In that time, was not even an discussion if WLKs are arcane or divine casters. Ammon Jerro, the most notable warlock(nwn2) was faithless without any deity or "patron" and was an warlock because warlock is just an "outsiderish" version of an arcane caster, just like an druid is an "naturish" version of an nature caster.
Non deities can't grant spells, doesn't matter how strong they are.
Think on WLK just like an "outsiderish" version of arcane caster, just like Druids are an "natureish" version of divine casters. Some quotes.
Note that divine casters had mechanics for losing powers like Oathbreaker paladin, warlocks have no such mechanics.Eldritch Invocations
In your study of occult lore, you have unearthed eldritch invocations, fragments of forbidden knowledge that imbue you with an abiding magical ability."
https://www.dandwiki.com/wiki/5e_SRD:Warlock
Some quotes from player manual
Quoting complete arcane(3.5)"... young elf in golden robes smiles warmly, weaving a magical charm into his honeyed words and bending the palace sentinel to his will"- Not Patron's will- Warlock's will
"Warlocks are seekers of the knowledge that lies hidden..." Doesn't seem like the warlock is being gifted the power so much as taught the methods on how to wield it.
"Drawing on the ancient knowledge..."
"...warlocks piece together arcane secrets to bolster their own power..." Again referencing the Warlock's own power being enhanced by universal secrets
"Warlocks are driven by an insatiable need for knowledge and power"
"...while poring over tomes of forbidden lore..." Again with the idea of studying eldritch secrets.
pag 5
"seeks to master the periculous magic that suffuses his soul"
"the fount of dark magic burning in their soul"
pag 8
"much like bards and sorcerers choose which..."