The reference implementation of BLAS is in Fortran, but these days fast implementations (using the same interface) tend not to be. OpenBLAS is in C and much of MKL is in C/C++. An individual widely-used package can be rewritten, much like improving a compiler, or giving it a new output, benefits lots of software all at once.

However, OpenBLAS still links to the reference LAPACK in Fortran, because it's mostly made fast by linking it to a good BLAS implementation. It could be rewritten, or converted to C with f2c. Although there's a long tail of useful but often niche Fortran software, some in Fortran 90 that f2c can't convert. Having a fortran compiler is still useful, and covers all of them at once. Potentially that compiler could compile to another language, to cover more and future platforms all at once (like f2c does). It looks like that's what's going on here, compiling fortran to wasm.