I don't consider it a "league" at all. It's just two battles. So I would still rank it just above Unova. Just because of the laziness.
A league battle with the main character can, by definition, not be a filler.
Two Battles? You have Alain vs Trevor, Ash vs the Altria Trainer, Ash vs Astrid, Sawyer vs Tireno, Remo vs Alain, Ash vs Sawyer, and Ash vs Alain. Not every battle has to be Ash vs someone.
And League Battles can be considered filler if it doesn't advance the plot or can be easily cut. It's simply padding out the series.
Doing some trivia counting, all the Leagues up to this date have an average of 7 episodes (counting non-battle episodes as well).
Indigo Plateau Conference: 7 Episodes
Silver Conference: 7 Episodes
Ever Grande Conference: 7 Episodes
Lily of the Valley Conference: 8 Episodes
Vertress Conference: 6 Episodes
Lumiose Conference: 7 Episodes
As we can see, no League Conference has ever exceeded 10-Episode mark limit. The Sinnoh League is the closest, but only because one battle (Ash vs Paul) took 3 episodes to be told. The next battle, by contrast, took about one to wrap things up.
Ash's Full Battles:
Indigo: 0
Silver: 2
Ever Grande: 3
Lily of the Valley: 2 (note: 6 vs 2 for Tobias)
Vertress: 1 (note: 6 vs 5 for Cameron)
Lumiose: 2
As we can see, Full Battles are very rare. And if we discount Tobias since his Battle was more of a 6 vs 2, there are a total of 9 Full Battles out 6 Leagues. 8 if you discount Cameron. An average of 1.333 Full Battle or 1-2 Full Battles in a given League; the rest of the League battles being 3 vs 3.
This tells us that Full Battles are not animator friendly and are reserved for big events. Also note that Full Battles during the Johto and Hoenn era were back to back. So 3 episodes were dedicated to Full Battles in Johto; 4 episodes were dedicated to Full Battles in Hoenn; 3 episodes dedicated to Full Battles in Sinnoh (4 if you count Tobias); 2 episodes dedicated to Full Battles in Unova; and finally, 4 episodes dedicated to Full Battles in Kalos.
As we can see, Kalos League isn't really shorter compared to the rest of the Leagues. It's about the same length. It only feels short because the preliminaries and semifinals were focused on Ash's future opponents, Sawyer and Alain respectively instead of Ash; while one non-battle episode is used to set up the Team Flare climax (which itself is 5 episodes long). There's also the fact the other Leagues never got Ash as far as he did in Kalos, usually landing in the Top 8. As such, many fans imagined how long the League would be if Ash wasn't knocked out, which extends it around 10 episodes.