The Sphinx configuration currently builds a single manual page, from the entire master document. In conf.py:

man_pages = [(master_doc, "towncrier", "Towncrier Documentation", [author], 1)]

The resulting manual page does not meet the conventions for section 1 of the manual ("User Commands"). (See the man-pages(7) manual page.)

The document should instead be in the manual section 7 ("Overview"), to describe the Towncrier system as a whole.