Are Christians even polytheists?
They would emphatically say no, but the Trinity suggests otherwise.
Three distinct entities, with distinct personalities, goals and natures: Father, Son and Holy Spirit. The Son forgives easily, the Father holds grudges for generations. It's an unforgivable sin to blaspheme against one, but not the others. Meanwhile, the description of the Trinity as essentially distinct aspects of a single being is similar to some interpretations of Hinduism. There is at least a superficial (and not at all radical) case to be made that Christianity is monotheistic in name only.
So would, or do, Muslims consider Christians to be polytheists? I don't know. But it seems they would need to for that verse to really apply.
Although... Christian teaching holds the Old Testament to be as sacred and inerrant as the New, and that has many examples of God (the Father) commanding His people to stone the unclean and murder the infidels (which at the time would be anyone not a Jew, because of course the God of Israel was, exclusively, the God of Israel, (until He wasn't)) and the Book of Revelations is almost entirely about a holy war between Christians and Everyone and Everything Else.
Yet, I doubt you would find the majority of Christians or Jews holding to such a violent and literal interpretation of scripture. Some, yes, obviously. In the US at least, apocalyptic Christian theology is big business (just see the Left Behind and assorted clones in any bookstore) and big politics.
Nevertheless, it is no more the case that all Muslims want the infidels dead (but only a few are willing to pull the trigger) than it is that all Christians want every woman on her period within the city limits to be stoned to death. The truth is, most religious people are hypocrites, and the world remains the better for it, and both Islam and Christianity could (and have, now and then) justify genocide and hate with their Scriptures if they wanted, because both religions come from a time when those were SOP for religions and societies.