Thank you for that correction. You are correct. It is obvious that Miriam was a Nevi'ah, because the Torah calls her so. The Gemara is teaching when it was that Miriam prophecized. This is evident from Rashi on that verse (Shemos 15:20), who says "Heichan Nisnav'ah..." -- " When did she prophecize...."

(See, however, the RAMBAN, RASHBAM, and the KLI YAKAR for alternate explanations of the P'shat (as opposed to the Derashah of our Gemara) of the Pasuk.)

All the best,

Y. Shaw