They have open sourced a development toolkit for making expansion card modules but the motherboard itself is not open source. You can, however get schematics if you are a "repair shop", under NDA, however they define that.

I hope the schematics leak, just like they do for many other laptops.

That just reminds me of Alienware’s open modular GPU cards or the open nanoSD cards from Huawei, both of which are open and nobody wants to make or use in their own products. It’s open but there isn’t any incentive for anyone else to make some niche parts for a single laptop brand.

Entirely? That's just for expansions which are essentially USB-C dongles.