You just answered your own question. The bible points out that it isn't possible for the elect to be deceived, yet we also see claims to be alert for false prophets. Are those warnings directed at the elect? I don't see anyone making that claim. When one sees an organization engaging in deception, it stands to reason those are not the elect of God.Then why do we need to worry about false prophets?
No one is more deceived than the father of lies, Satan himself. Those who practice deception are no different.