I'd check and make sure. Sounds to me that little rule may apply to a legally fitted and engineered seat rather than something bolted down in the cargo area of a commercial vehicle.

I had a couple of rear facing seats in the back of mine about 14 years ago. They were bolted down with oversized plates under the floor and fitted with seatbelts and everything. Inspection time came around and because there was no engineering certificate it failed inspection and I had to remove them.