I looked at, at least 8 different moddio games. They're all top down Newgrounds games with painfully obvious similarities. The only exception was one where you control a fish. In that one the water splash animation occurred on the grass leaving me to believe the game was pushing the envelope of what the engine could do.

One trick pony engines worked in the 90's because something like an FPS required rare personnel to develop given the limited hardware. That's why you'd get weird kids stuff like Nerf Arena Blast (Unreal 1) and Super 3D Noah's Ark (Wolfenstein 3D engine). Eventually you had Renderware, but that was like the Unity of the 2000s.

There's a world of JS game engines, more advanced, for free today https://github.com/collections/javascript-game-engines

Moddio has built in multiplayer server support that seems pretty intuitive, that's definitely something special. But a screwdriver that makes Stick RPG clones isn't very profitiable.