The way Claudius says this, it sounds as if he’s watched both these guys in a show or something. Granted, I have a heavily theatrical sensibility – so of course it sounds like he’s seen them both onstage to me.
Sports are also a thing people go see. It is a spectator experience. It’s not entirely impossible that Claudius could have watched both these guys compete.
When have either Laertes or Hamlet had time to participate in sparring? Hamlet’s been to “England” and Laertes has been in France. Previously, it was funeral and wedding time at Elsinore. Were there games as part of those celebrations?
When exactly has Claudius seen them both?