Sure, a lot of Matlab is in Fortran (and a lot of other software as well, though you would not know from the outside), but you don’t write in Fortran when using Matlab.
Microsoft BASIC-80 (around the same time) was written in assembler. You also don't write in assembler when using BASIC-80. But, BASIC-80 would be extended by assembler, in the same way that MATLAB would be extended by FORTRAN.