It doesn't make sense that that is the reason. Surely C++ has way more packages. So Go would be a bad choice if package availability was a very important concern.
> Go is simple and straightforward, but I often wish it offered more type safety. Rust is very nice to work with (at least for single threaded code) but it's too broad and complex, sometimes painfully so.
It seems clear that the author really likes aspects of both Go and Rust and desires something between the two. Check out the complexity vs type-safety illustration at the top of the page, with Borgo placed between the Gopher and the Crustacean just before the complexity curve gets steep.
They're basically building their personal ideal version of Go with inspiration from Rust.
The type system lets them reject the programs they want to reject, but if a program is valid in the type system it can, in large part, emit essentially the same go code sans the types. I mean, I'm making some assumptions here, but that's typically the reason.