Could you please elaborate on this a bit more — i couldn’t find the direct answer to my question in the Umer Sb comments you shared above.
When we accept the principle that anyone who calls themselves a Muslim and insists on it cannot be declared a disbeliever—even if their beliefs or actions contain elements of shirk (polytheism) or outright disbelief—then a question arises:
Does this not open the door for anyone to claim whatever they wish, while continuing to call themselves a Muslim, and still be recognized as such?
If you say such a person would technically be a non-Muslim, then for example, if a person were to say, “I am a Muslim, but I do not believe in Allah” or “I am a Muslim, but I reject the Qur’an,” then technically such a person is a disbeliever—even if they verbally insist that they are a Muslim.
But this is precisely the argument of those who say: since such people hold beliefs of clear disbelief, they should indeed be called disbelievers. What a person says with their tongue is not enough; if their beliefs are kufri (disbelieving), then they are to be regarded as disbelievers—since technically they are denying the command of Allah, which in essence is the same as denying Allah Himself.