My understanding from that particular speech is that it was something that would happen much later - it had nothing to do with the current "plan."

As for how he would have discovered it? Loki's gifts are cunning, wit, and trickery - how did he get the jotuns into Asgard? I don't think it would be very difficult at all for him to sniff out someone's secrets, if he wished to know them. I do not think Clint was his only source for information.