This is somewhat deconstructive.

So you need the solo developer not to contract out or buy in existing music, graphics, 3D assets, animations, marketing, or you won't call them a solo developer.

Right, so do you also need them to create the 3D engine or are they allowed that off the shelf? Oh, they need to make it themselves. You're strict!

Ok, so they're allowed to write for a platform? Oh, no they're not, that's relying on other people's code.

And writing in an existing language? Tsk tsk tsk. Got to invent the programming language yourself, otherwise you need to list the entire GCC/LLVM team as your collaborators on the game.

They have to create their own silicon too, it's cheating to rely other people's chips, how can you call yourself "solo"?

Are they allowed to sell it on Steam or do they need to build their own store and payment networks? Heck, should they get themselves accredited as a payment network. Oh, and as a bank.

And presumably, if the game needs to be translated to any language other than the developer's own, they have to do that translation themselves, right? Not rely on experts in that language. Can't really be a "solo" dev that way, can you?

And so on.

Building a game involves effort, and millions of decisions. Is the gameplay right? Is the story right? Are the graphics right? Design the characters, the levels, the world. Make the game run. Make the game available?

I can accept that solo developers will sometimes make the graphics/music/"assets" themselves, sometimes buy off the shelf, sometimes pay others. But unless they hire that person full time to collaborate on the game... they're still the solo developer.

They will definitely lean on existing 3D engines, libraries, plugins, font engines... and that reminds me, I've almost never seen a game developer design their own fonts. These reusable components can be used in games, and some are even intended to (e.g. engine plugins). But do they define the game experience? Generally, no. That's on the game developer.

Here is Jonathan Blow's placeholder art for Braid: https://upload.wikimedia.org/wikipedia/en/2/2f/Braid-art-1.j...

Here is how good David Hellman, the artist he hired, made it: https://upload.wikimedia.org/wikipedia/en/2/2e/Braid-art-2.j...

David Hellman is credited as the game's artist, but it's still effectively Jonathan Blow's game from top to bottom.

Train sim created by just one person is being called the best ever made