Just to be clear - that's Ezreal's interpretation of the League. He's new, and the story is mainly from his perspective. He's not omniscient... at least as far as the League is concerned.
I'll try and clear that up in future revisions!
Like, I can understand if a champion is summoned from another world that they wouldn't know any summoners, though if they stay near the league there would be a chance of meeting their summoner yes?