Because if you prefix a wikilink link with a single colon it always works as a link regardless of namespace. And until the recent MediaWiki change two colons worked too, so if a template wants to accept both prefixed and unprefixed links, it can append a single colon to the front.
I see that the linter has been deployed, but it doesn't seem to be picking up errors that result from template transcriptions, e.g. https://zh-yue.wikipedia.org/wiki/%E8%A9%A9
(Neither this page nor the template the caused the error has been changed since the MediaWiki breaking change.)https://zh-yue.wikipedia.org/wiki/Special:LintErrors/multi-colon-escape
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