The Contradiction with the Qur'an in This Regard
Treating a polytheist as a Muslim and referring to him as such inevitably leads to conclusions that contradict the statement of the Qur'an.
Thus, following the previously quoted Qur'anic verses, the next verse clearly states that the path of the mentioned prophets عليهم السلام is the true guidance:
﴿أُولَئِكَ الَّذِينَ هَدَى اللَّهُ فَبِهُدَاهُمُ اقْتَدِهْ قُلْ لَا أَسْأَلُكُمْ عَلَيْهِ أَجْرًا إِنْ هُوَ إِلَّا ذِكْرَى لِلْعَالَمِينَ﴾
Those are the ones whom Allah has guided, so from their guidance take an example. Say, “I ask of you for this message no payment. It is not but a reminder for the worlds.”
[sura al-An'am, 6:90]
These prophets are therefore the rightly guided, and their guidance should be taken as a model and a guiding principle.
However, as has already been demonstrated, this passage in the Qur'an clearly states that the guidance of these prophets consisted precisely in the abandonment of shirk.
If, on the other hand, they had committed even a single polytheistic act, all their good deeds would have been lost. In that case, they would have been completely misguided and by no means rightly guided.
This meaning is also found in other places in the Qur'an, such as:
﴿وَلَقَدْ بَعَثْنَا فِي كُلِّ أُمَّةٍ رَسُولًا أَنِ اعْبُدُوا اللَّهَ وَاجْتَنِبُوا الطَّاغُوتَ فَمِنْهُمْ مَنْ هَدَى اللَّهُ وَمِنْهُمْ مَنْ حَقَّتْ عَلَيْهِ الضَّلَالَةُ فَسِيرُوا فِي الْأَرْضِ فَانْظُرُوا كَيْفَ كَانَ عَاقِبَةُ الْمُكَذِّبِينَ﴾
And We certainly sent into every nation a messenger, [saying], “Worship Allah and avoid taghut.” And among them were those whom Allah guided, and among them were those upon whom error was [deservedly] decreed. So proceed through the earth and observe how was the end of the deniers.
[sura an-Nahl, 16:36]
This verse would also undergo a reversal of meaning. A polytheist has associated others with his Creator in worship, meaning he has worshiped the taghut and has not implemented the turning away from it. Those who followed and accepted the guidance of the prophets completely refrained from worshiping anything else.
Ultimately, this very matter was the guidance mentioned in the verse, which was granted to the people described therein.