In episode 5, The Empire Strikes Back, Emperor Palpatine seems worried that Luke could become a Jedi.
VADER: What is thy bidding, my master?
EMPEROR: There is a great disturbance in the Force.
VADER: I have felt it.
EMPEROR: We have a new enemy - Luke Skywalker.
VADER: Yes, my master.
EMPEROR: He could destroy us.
VADER: He’s just a boy. Obi-Wan can no longer help him.
EMPEROR: The Force is strong with him. The son of Skywalker must not become a Jedi.
VADER: If he could be turned, he would become a powerful ally.
EMPEROR: Yes. Yes. He would be a great asset. Can it be done?
VADER: He will join us or die, my master.
By this time, both Sith Lords believe all the Jedi are dead. Even Grand Moff Tarkin believed that Vader is the last of their kind, and that was back in Episode 4.
VADER: He is here.
TARKIN: Obi-Wan Kenobi? What makes you think so?
VADER: A tremor in the Force. The last time I felt it was in the presence of my old master.
TARKIN: Surely he must be dead by now.
VADER: Don't underestimate the Force.
TARKIN: The Jedi are extinct. Their fire has gone out of the universe. You, my friend, are all that's left of their religion.
Did Palpatine seriously believe Luke could become a Jedi?
Unless Palpatine believed some Jedi remained in hiding, he should have assumed that nobody could have trained Luke. It took years of training to become a Jedi, so he should have assumed that even though Luke was very sensitive to the Force, nobody could train him to hone his innate abilities into real skills.
Please provide answers with quotes from canon sources. Is there a canon source that says Palpatine believed some Jedi were still alive and capable of training Luke?