That specific script is built on work with no explicit license terms. I’ve noted that in the notes at the top, and presumably anyone who is using it is comfortable with that. But it does make it unsuitable for inclusion in the freePBX core.
One of the goals of Asterisk 19 (due out in the fall) is improved support for TTS and STT. Once we see that take shape, ‘proper’ FreePBX support will follow.