You can render remarkup using this rule without having a meaningful relative base URI, e.g. on some other object type or via remarkup.process in the API. This isn't necessarily a meaningful operation and the result may not be useful, but remarkup generally tries not to fail loudly: if you copy/paste a block of text from somewhere that happens to have some substrings which aren't valid, the desired behavior is generally for your text to be processed in some best-guess-at-sensible way -- and usually emitted unmodified (see also discussion in D21713).

For example, if you write or paste a piece of text which includes @gmail.com, but do not have a user named gmail.com on your install, it is not likely that you intend to mention a user. The current desired/actual behavior is to render your input literally (@gmail.com), not fail loudly ("Error: unable to render remarkup because it contains an invalid mention, no such user 'gmail.com'!"), with a hint in the preview only that the mention isn't valid.

The particular behavior of [[ ./relative/path ]] in a non-Phriction context today is questionable (you get a link to Wiki document /w/dot/relative/path/, i.e. the default slug for a Wiki page literally named ./relative/path), but this change doesn't affect it.