Even if that was true - there was multiple times we could have used both of them, so a load of rubbish IMO.
As to why Sanson didn't work out here, no idea, he's clearly a capable footballer. My take is probably:
Injured a lot of the time under Smith, hard to get into a proper rhythm and properly adjust to English football.
Under Gerrard - the clown had no idea how to set up a midfield and Sanson wasn't one of his favourites. Steven was also petty enough to hold that bottle kick against him.
Under Emery - by this point he was desperate for game time and probably low on confidence. Easy decision to loan him out and get him game time.