It basically means that he would have been in your Youth Intake when you got it but he signed with City before then. To be honest i don’t think it affects anything in game as I’m pretty sure it’s more of a random event than anything else.
Or to put it another way, he was always going to be in City’s intake but the game has randomly said that he was poached and sorted out transfer details to show that.
Additionally I may be wrong but if you look up the player history it should say that he was with United and then went to City to show that