I suspect it's an oversight more than anything else. If you dig into the export_descr_buildings.txt file, you'll find the list of buildings and what is required for/provided by each. Legionary cavalry does not require the marian reforms, while the early and late legionary cohorts do. I think the legionary cavalry probably was supposed to require the reforms, but it wasn't put into the file. Personally, I've added it to the file so I can't build them until after the reforms.