Next, wrong about that. Warlock Fiend patrons are not deities...full stop. If a DM wants to say you are the warlock of a deity but you still play as Fiend Warlock that's homebrew, which DMs are allowed to do but BG3 as a videogame can't accommodate that.
Asmodeus would like a word with you...
Asmodeus is both a greater deity (as of Descent into Avernus) and can be chosen as a fiendish patron of a warlock pact (as stated in the PHB). An exception, sure, but still possible.
Think that applies to Dendar and Tharuzdin too.
Also Vlaakith, although she is probably a lesser deity, but 5e doesn't make a rules distinction.
And in the Explorers Guide to Wildemount (WotC) the table of "Lesser Idols" has 10 beings who empower both clerics and warlocks. + Vecna, who is Greater Deity in Exandria, and also a warlock patron.
