Template manager has a button which opens the extensions site. This should be replaced with the internal dialog.
Heiko Tietze committed a patch related to this issue.
It has been pushed to "master":
Resolves tdf#137477 - Access templates via tight extensions
It will be available in 7.1.0.
The patch should be included in the daily builds available at
https://dev-builds.libreoffice.org/daily/ in the next 24-48 hours. More
information about daily builds can be found at:
Affected users are encouraged to test the fix and report feedback.