The //go:xyz comments are an established pattern in the Go tooling.
(My personal theory is that early go had a somewhat misguided idea of simplicity, and preferred overloading existing concepts with special cases over introducing new keywords. Capitalization for visibility is another example of that.)
(Though for the record, talking about alternative implementations when discussing Go is kind of a funny joke.)
But to be fair to alternative toolchains, TinyGo and TamaGo are also a thing.