It also looks like Lance will use a Hydreigon. It could be Iris's instead, but the framing implies that it's Lance's. Should this be the case, then it means everyone will be using three Pokemon.
EXCACHAD>>>>>>>>>>Haxorus
Even if you go by games Diantha, although easier due to how XY is built, is actually somewhat higher level than Lance (levels 64 to 68 vs levels 58 to 62) and could quite plausibly defeat him.
And this is without getting into how if we take a look at the teams each of them uses, Diantha has far more counters to Lance's team than Lance has counters against hers. I mean, girl's got two Rock-types, one of which is also part Ice, meaning she can deal super-effective damage to the entirety of Lance's team with only a third of hers. And Goodra and Gardevoir are also a thorn in Lance's side for obvious reasons.
On the contrary, the writers being able to cram Ash vs Hop and the Masters 8 introductions into the Leon vs Alain episode leads me to believe that there will be some additional stuff happening in this episode too.
It's quite possible the Leon/Alan stuff spills over into this episode too. I doubt we need an entire episode of Lance vs. Diantha since neither are major characters, so I could see some stuff continuing here.
To be fair, the Kalos Elite Four and Diantha never got a difficulty spike when rematched in XY. Diantha's team could only really reach Level 80 through the Battle Cheateu's Black Writ of Challenge, but this rematch at her strongest is only against four Pokémon, and she's faced as a Grand Duchess instead of Champion (at least she keeps her amazing battle theme here).
To strengthen your point, The Fiercely Fought Don Battle! Tsutarja VS Komatana!! went over into the next episode despite the majority of the battle having been done in its titular episode; there are other examples I'm sure, but this is what immediately came to mind.
