Quote: "make it obvious Steam has nothing to do with these limitations."
Actually, it has, but it took my brain also a while to figure it out. Steam was used for Multiplayer, but the Steam multiplayer game created with GG was always Gameguru, not the (non-existent) StandAlone. So it had (has) towards the Steam SDK Multiplayer component always the AppID of GameGuru. You never played [YOURGAMENAME] in Multiplayer, but always GameGuru itself with your "Game" simply being the map / level. This kicked in the requirement for joining players having GameGuru themselves (and I bet on the same version, too).
It kicked also in the requirement of "stock assets only" for two reasons:
1. A system detecting which files the joining player has and which need to be transferred is quite sophisticated MP code, and with the Map sizes involves would have triggered Gigabytes of Traffic in a single game.
2. Legal & ownership reasons: Such a system would have transferred all assets towards the joining players, for reason above GameGuru owners themselves. Any self-made, bought or DLC content would have been - at least temporarily - on the joinings player hard drive. Even if you add a clever deletion routine, that`s a legal booby trap.
So, if Max can actually create Stand Alone Multiplayer, custom assets must be of course supported unless they trigger behaviour that cannot be MP synched with the netcode. Own static entities, textures, terrain etc must be possible.
"I am a road map, I will lead and you will follow, I will teach and you will learn, when you leave my sprint planning you will be weapons, focused and full of JIRA tickets, Hot Rod rocket development gods of precision and strength, terrorizing across the repository and hunting for github submits."