I searched the wiki and expert takes thoroughly, but I can't find any mention of George Kittle issuing a "cryptic warning" to Brandon Aiyuk suitors in the available content. That specific quote or story doesn't appear to be covered in our database.
What the wiki does cover on this front: Kittle is currently deep in his recovery from a torn Achilles (ahead of schedule, targeting Week 1), and the 49ers have essentially moved on from Aiyuk — voiding his guarantees, signing Mike Evans, and with GM John Lynch signaling a split is inevitable. As of late June, ESPN's Jeremy Fowler reported Aiyuk is "essentially untradeable right now" due to administrative hurdles with his reinstatement (from the Brandon Aiyuk page). Sorry I couldn't track down Kittle's specific quote for you!