What's wild about it? It's one person, and they probably mainly use a Mac. If they used Windows, it would be Windows-only at this stage. I have lots of projects that only run on Linux. Clearly, if the project succeeds and gains tractions, the other platforms will come.
It's totally fine if it just supports one platform. Just don't say it's cross platform when it's not.
With Linux you can dockerize it and make available for all all platforms. With Mac you only reach people that have access to Mac.
I mean for a personal project they can use whatever they want. Just and odd choice to start with if you want the project to gain traction.
It assume it will severely limit its uptake.
If I had to pick one platform to build on, it would be linux. Pretty much every platform has the ability to run linux executables (via Docker, among others).
If this is a personal project, that's cool, but then I really don't care that much. On the other hand, I think an electron-like with something like Bun could be really useful.