Also see Qamoos al-Muheet, Misbaah al-Muneer , Taaj al-‘Aroos etc.
Some have also explained that kufr is shirk because it is essentially taking one's own whims as a god besides Allah (see 25:43). And there
There are other examples within the Quran and Ahadith where shirk and mushrik are used for disbelievers in general, not justKufr rather than self-admitted polytheism:
Consider the following verse:
ولا تأكلوا مما لم يذكر اسم الله عليه وإنه لفسق وإن الشياطين ليوحون إلى أوليائهم ليجادلوكم وإن أطعتموهم إنكم لمشركون
And do not eat of that upon which the name of Allah has not been mentioned, for indeed, it is grave disobedience. And indeed do the devils inspire their allies [among men] to dispute with you. And if you were to obey them, indeed, you would be associators [of others with Him].
This verse was revealed when some of the Quraysh argued with the Muslims saying that it was lawful to eat dead meat (carrion). They argued that the Muslims eat what they kill themselves and what their own hunting dogs and hawks kill - so they should also eat what Allah Himself kills (i.e. animals which die naturally). This verse says that if we were to obey them (i.e. consider carrion to be lawful) then we would become Mushrikeen (i.e. would commit shirk). And it is obvious that considering carrion to be lawful is not polytheism but rather it is Kufr. And it is called Shirk even though the person will not admit to belief in multiple deities.
Also consider the hadith:
بين الرجل وبين الشرك والكفر ترك الصلاة
Between a man and Shirk and Kufr there stands his giving up the Salat
— Muslim
It is obvious that abandoning salah (i.e. not considering it obligatory) is Kufr, and it is called Shirk even though the person does not express belief in multiple deities.
And there are examples within the idolatersQuran and Ahadith where mushrik is used for Jews. For example consider the verse 2:135 - it is apparent that here Mushrikeen means all disbelievers and especially the intent is to say that Abraham was not a Jew or Christian. And it is also understood in this, this and this hadith that Jews are being referred to by calling them Mushrikeen. And scholars too will often in their writings refer to the Jews as Mushrikeen such as here.