cable/sat and antenna all get the same feed from the same source. So technicaly they can be exactly the same (and even in theory anyone can be higher).
But to understand the topic the best way to start is from the beginning. Let's say someone is watching Hockey Night in Canada (since we are Sat and in Canada). CBC sends a "high quality" feed to all the stations from the arena, the station gets it and adds local advertisement and stuff and you get your CBC station (for example in Toronto I think it is ch.5 for OTA) now technically it could be possible to get a direct signal to the cable/sat companies and not be limited by the OTA BW/specs, but what really happens is that the Cab and sat companies get that same OTA signal. Since they work off that channel they are limited by that channel. Now they get CBC Toronto as well as many other channels and they need to send them down the pipe to you and everyone else. So what happens is the cable /sat companies take that signal and re-compress it and that is done unevenly, some show/channels are compressed more than others depending on several factors.
That is why people talk about OTA being higher quality. Is it necessarily always there? no, is it a lot or a bit when it is there? that depends on the difference and how much you care.
I think the biggest reason people talk about it is that in the old analogue days the cable & sat companies had fine tuned directional antennas to get the signal and so Joe with his rabbit ears was getting snow, ghosting ... while the guy receiving cable/sat had a clear image. This has engrained the notion in many that cable and sat must look better. So people feel the need to point out that not only is it not true but that the opposite is true.