At this point it feels anachronistic to have a flash game that feature somewhat recent characters. It seems like this should be an older game that just recently got updated with konosuba characters, not a completely new game.
flash is still quite popular, might not be the monolith it once was, but it is still used quite regularly, there are just so many other games using different engines it may be hard to find.
One very successful and good game that uses flash is Crimson keep. The major benefits being more fluid animation, low file size, and unique aesthetic (exploration, instead of the same rpgm style). It could very well have been done in rpgm, but that would one immediately make the file size large, have animation limitations.
Since flash is designed for animation, where as rpgm is more for pixel animation so doesn't perform the greatest when doing sex animations, while possible still not the best), and the pixelated aesthetic is not optional unless you heavily mod it which isn't worth the effort imo (and the games that have are abandoned afaik).
If you want a visual of how alive flash still is for porn games, just go to latest tab and filter flash for the engine.
Should probably link to the example:
https://f95zone.to/threads/33488/