You are technically correct but practically wrong. Most Trad chapels aren't big enough to have a properly sized sanctuary and, room between the sanctuary and pews for things like an advent wreath. Trad chapels typically have devotional candles lit by the laity in the sanctuary, and that's against the rules as well. Lots of rules are not followed for practical reasons...the building are too small.
But in the grand scheme of things, the advent wreath is not part of mass, it's not part of the liturgy, because it's prayers are done before mass. This is the more important rule and i've never seen it broken.