I believe the creator of blastarena.io ran the PlayCanvas engine on the server using a headless browser to do the verification (PlayCanvas also has a ‘no render’ flag that you can use for performance).
The alternative for a real time multiplayer game is to have the logic of the game completely independent of the engine so it can be ran separately (i.e on the server) and the engine framework is just used for rendering, audio and passing input.