The Mister Multisystem appears to be a great improvement over the de10-nano. LGR did a video on it:
It's not the first time I see this confusion but I'm a bit surprised by it because they also make it very clear that "A DE10-Nano is required to operate"
The "mister multisystem" (which is a proprietary design) comes across like a slap in the face. Don't support blatant opportunistic profiteering.
E.g. the MiSTercade https://misteraddons.com/products/mistercade or the MiSTer mini ITX Ironclad Plus https://www.d3fmod.com/mini-itx-ironclad-plus/
None of these are trying to lock people into their solutions, they're just providing other options.