The parser for the .mplstyle files is eager when identifying comments. A # always demarcates the beginning of a comment, therefore it cannot be part of an rcparam.

Reference: #6907 (comment)
The reader function: https://github.com/matplotlib/matplotlib/blob/af16ac8/lib/matplotlib/__init__.py#L1055
Example style file: https://github.com/matplotlib/matplotlib/blob/master/lib/matplotlib/mpl-data/stylelib/classic.mplstyle#L183