I'd disagree that the terms are synonymous. Free plans probably could mean printed out single sheet plans with renderings. Open Source usually means the legal freedom to change, update and share whatever it is. To me, the source of the plans is provided so that someone could change/update the plans as they wish. In the context of architectural plans this could be the files in something like Fusion 360 if that were actually used for full house plans (I don't think it is).

Edit to add: I doubt an architectural firm would give their source files to plans away for free. They put a lot of effort into creating them, its their business after all.