It assumes he does IF he has time to prepare. Which he may or may not.
A 15th level party has access to mighty divination spells, has probably fought demons and devils before, and should damn well be prepared for this kind of thing (especially invisibility). If they're not, then they don't deserve to beat a foe 5 crs above them.
I don't call owning silversheen and oil of bless weapon 'advanced knowledge'. I call it basic adventuring protocol for levels 11-20. If you're coming up against a major demon as a BBEG (he IS 5 crs above you, remember?), then you SHOULD have some inkling of what he is.
Flight is passe at this sort of level. If parties are still being stumped by flight, then they deserve to get dropped.
Greater Teleport will help set him up as a recurring BBEG, but it's unlikely to help him take on the party, unless their quest is to slay him (in which case they're probably prepared).
Either he's fighting to keep them away from something (which he won't succeed at if he teleports away) OR he's fighting to kill them (which... again...).
Regardless - you're complaining that going into a fight 5 crs above you, which you've given a chance to prepare, and know nothing about is going to be tough.
Well, duh.