No. Measurably not true.
Dungeons drop less because they’re splitting loot between you and the 4 nonexistent other players. Unless they’re follower dungeons, then you get it all no matter how fast you nuke everything (expect that bit o fun detected to be nerfed in 12.0).
Quest mobs don’t drop 100% because they want you to kill more than 6 mobs without saying exactly how many. You’ll remember the times RNG put you on the wrong end of that. Most famous was the one collecting 4 (?) zhevra hooves. A number that low means the drop chance is also kinda low, and the RNG variance can be huge.