They do indeed have far more spell slots than they should; as Aishaddai mentions, you can raise a variety of them with Glut to see this, since they come up with their full ability set. It's not just one or two extra; it's more like a level 5 character with eight 1st level slots, six 2nd level slots and four 3rd level slots... while others have custom homebrew 'abilities' which emulate 1st or 2nd level spells that they can use unlimited as bonus actions.
This is NOT at all the way it goes in TT (and if it is you should probably get a new DM); enemies play by the same rules as you except in very specific exceptional circumstances.
