Probably not. @container is a relatively new thing introduced to LESS, and the library XenForo uses for LESS hasn't been updated in many years:
less.js ported to PHP. Contribute to oyejorge/less.php development by creating an account on GitHub.
github.com
In fact the developer has deprecated/archived the library and recommends people switch to a "well-maintained fork":
Less.js ported to PHP. Mirrored from https://gerrit.wikimedia.org/g/mediawiki/libs/less.php/ - wikimedia/less.php
github.com
It probably makes sense for XenForo to update/switch the LESS library they are using (that's a different thing though), but even the new recommended one doesn't yet support @container. See:
As reported in #78 (comment). I made a very rough "fix": Gappa@d255214 It kind of works, but hijacks most of the code for media queries, upon which container queries are based, so it makes at least...
github.com