I guess I could have worded that part better. My point is that if you get a hardware RAID card, those features are advertised and supported by the hardware vendor. You can of course get a cheap hotswap-capable controller card and run mdraid on that, but realistically it's probably going to be untested and unsupported unless you're buying it in a preassembled NAS or something like that.
Hardware raid does NOT give you any hotswap and/or online array rebuilding that doesn't ALSO exist within mdraid. Yes, mdraid is happy to allow you to add/remove volumes from the array and/or rebuild the array entirely online with no downtime just like hardware raid.