It's multiplayer. But only with Steam backend. A backend is needed for match making so player can find each other. Also Achievements and stats and such cool features need a backend. So, if you like the Itch.io version, just come to discord and grab a free Steam demo key.
I have a support ticket open at Itch, if it is possible to deliver steam keys to itch user who downloaded my game. But I guess this is only supported by itch for payed games at itch.
Come to Discord also if you want to give feedback.