Is someone who jokingly calls a Muslim a "disbeliever" for delaying prayer – while unaware of the hadith "whoever calls a Muslim a disbeliever is himself a disbeliever" and intending to reprimand and warn the Muslim – considered a disbeliever?

It is not permissible to joke by saying "O disbeliever" to a Muslim, and disbelief does not occur by this statement. Rather, its meaning is that the accusation of disbelief has returned upon the speaker, as if he has disbelieved himself. The hadith admonishes a Muslim from saying that to his Muslim brother. The verified position is that whoever says that to someone whose Islam is known without doubt, disbelieves by it. So the meaning is that the accusation of disbelief has returned upon the speaker, not disbelief itself. This is supported by the fact that in some narrations: "Disbelief has become incumbent upon one of them."