Is he who joked with a person who said "The servant of the people is their master" by saying that the one who said it is insane, then doubted that the speaker might have been the Messenger and sought forgiveness, considered an infidel?

The questioner's statement is not disbelief (kufr), as he did not intend the Prophet (peace be upon him) by it. And the hadith: "The master of the people is their servant" is weak; its attribution to the Prophet (peace be upon him) is not established. And Allah knows best.