The tag is present in Lintian version 2.104.262. That is the most recent version we know about.
The given source file is licensed under GFDL, but without any precision about the presence of invariant sections, front-cover or back-cover text.
GFDL license explicitly requests you to document this non-presence.
Refer to https://wiki.debian.org/qa.debian.org/gfdlinvariant and https://www.debian.org/vote/2006/vote_001 for details.