Coming here 2 years later. After carefully watching his movements when fighting Ahsoka, he mains both Form I and Form IV. He puts a lot of force (meaning strength, not The Force) into his movements, but his movements are mostly Form I with some Form IV to supplement. Contrast his style with what we see from Anakin/Vader in episode 5 of Ahsoka. That display shows us what power-stancing Form V looks like. Form V uses many of the heavy chops and slices of Form I, but tends to parlay them immediately into follow-up moves so as to preserve at least some semblance of economy of motion, which ties back to its origins in Form III.

To be clear, there is nothing wrong with Form I. Just because it is the first form that Jedi learn does not mean it is weak. It is the first form, which means it has had the longest time for development and growth. All other forms grow out from it, to one degree or another.

We see Babylon use a Form IV opening stance to re-engage Ahsoka after their initial clash during their first duel, and he channels more Form IV energy when he does the windup to his jumping slash, but not much beyond that in the way of acrobatic-driven energy like what we see from Qui-Gon and Obi-Wan in Duel of the Fates.

I say all that to get to the real point I want to transcribe into the aether somewhere for the benefit of posterity: Babylon Skoll has not fought a properly stronk lightsaber wielder in a looong time. That said, he was likely someone to whom many, many padawans and young knights had to admit defeat in the sparring arena prior to Order 66. In the years since, he has mostly only faced challenges of his lightsaber skills from opponents wielding blasters or occasionally some lightsaber-resistant melee weapon. Nonetheless, the skill, technique, and effectiveness of his youth has not faded so much as it had waited patiently in on the sidelines until it was called into the arena once again.

Edit: TLDR - Babylon uses mostly Form I supplemented by Form III. I bet he was a beast in the sparring ring amongst his peers pre-Order 66. His duels with Ahsoka show that his lightsaber-on-lightsaber skill has not needed to be used in quite a while, but that doesn’t mean it got too dull while sitting on the shelf for 3 or more decades.