The tag is present in Lintian version 2.104.318. That is the most recent version we know about.
The previous version of this package had a distribution of "experimental", this version has a distribution of "unstable", and there's apparently no comment about the change of distributions.
Lintian looks in this version's changelog entry for the phrase "to unstable" or "to sid", with or without quotation marks around the distribution name.
This may indicate a mistake in setting the distribution and an accidental upload to unstable of a package intended for experimental.