Separate page for comments using permalinks and add_rewrite_rule

WordPress rewrite rules by default are no longer stored in the Apache configuration, but are handled by WordPress itself. This was probably not the case when that forum topic was written (5 years ago), but currently only “external” rules end up in the Apache configuration, all others are stored in the database and handled by the WP::parse_request() function.

This means you should leave off the Apache-specific parts in your add_rewrite_rule() call. The following code looks better:

add_rewrite_rule(
    // The beginning `^` is added by WordPress
    '([0-9]{4})/([0-9]{1,2})/([0-9]{1,2})/([^/]+)/comments/?$',
    // `$1` should be replaced with `$matches[1]`
    'index.php?year=$matches[1]&monthnum=$matches[2]&day=$matches[3]&name=$matches[4]&comments=1',
    'top'
);

When I tested this with my Rewrite analyzer plugin, I noticed that comments is not a standard query variable, so remember that you will have to handle that too.