Is likening a group of Muslims to "Arabs of '48" – due to a difference in opinion – considered an act of disbelief (kufr), given that it likens other Muslims to Jews?

You are not considered an infidel for what you said, because you might not have intended to liken them to Jews in their disbelief, but rather meant that they are upon falsehood. The hadith (definition): "Any man who calls his brother 'O disbeliever,' one of them will surely have committed it; if the person is as he said, then it is so, otherwise it will return to him," is understood to refer to minor disbelief, as evidenced by the Prophet's (peace be upon him) saying: "And cursing a believer is like killing him, and whoever accuses a believer of disbelief is like killing him." And killing is not disbelief.