All double-hyphens ("--") become n-dashes when posting to the forum. There isn't a way to escape a double-hyphen to prevent this. It is a "feature" of the core bbcode code. Double-hyphens are often used in python command line arguments.
A work-around is possible by modifying pdxcg/pybb/defaults.py, line 62 to add the argument:
The new line is:
bbcode_parser.add_simple_formatter('code', '<pre><code>%(value)s</code></pre>', replace_cosmetic = False, render_embedded=False, transform_newlines=False, swallow_trailing_newline=True)