Here's a 'Behind the Doodle' about creating the game, and at one point I see they're using what I think is Adobe Animate for the level design and sprite animation (it says Animate in the top left bar), but beyond that, I didn't notice anything that might give a hint as to what the game's tech stack might be.
Might be Haxe, since it's kind of a successor to Flash like Animate is, and there's a Haxe library to support the Adobe Animate texture atlas format. Haxe games also tend to have retro pixel art styles.
https://www.youtube.com/watch?v=hy7tHQUR3TM
The video is pretty interesting and has a lot of concept art and storyboarding in it, btw, I recommend checking it out.
