Many jewish/rabbinical sources state that it is.
"it appears that a legal opinion[that islam is not idolatry] based on only one source (the Rambam) is being presented as the accepted mainstream position of the religion."
"the assertion that there is “no denial of Hashem in their religion or anything else that should cause them to be considered idol-worshippers” completely mischaracterizes the opinions of leading Jewish legal authorities. A significant and complex issue is being presented simplistically, and as a result many observant Jews receive a misimpression pertaining to their religion’s outlook."
" researcher Menachem Gottlieb of Jerusalem, clearly lays out the opinions of leading Jewish scholars throughout the generations "
"Rabbi Nachshon Gaon, one of the Gaonic legal authorities and head of the academy of Sura, in modern day Iraq, from 875 to 882 CE. He declared: “So we see the Ishmaelite (i.e., Arab) today is an idol worshiper even though he does not realize that he worships.” He further cited the rabbinic teaching that Mecca is one of five places in the world where idol worship goes on constantly."
During the 11th-12th century CE, the R”i Migash, as quoted by the Meiri, ruled that the Muslims continued in the practices of the pre-Islamic pagans.
Ibn Ezra wrote in his commentary to the Book of Daniel (11:30) that, “The men of Mecca did not turn to his [Muhammad’s] religion until he swore to them that he would not remove the Markolis worship, and it is not necessary to elaborate.”
In his commentary on the aforementioned Talmudic ruling from tractate Sanhedrin, the Ramah (12th-13th century CE) specified regarding the pillars of Markolis in Mecca that “since its worship is in throwing, when one throws, he is guilty [of idol worship].” In this way he clearly ruled that in throwing rocks at the pillars, Muslims were engaging in idol worship.
the Ritva (13th-14th century CE), who said that while Muslims are monotheists, their religion is considered “complete idol worship.”
The Ritva’s ruling was directly cited by the Radbaz (15th-16th century CE)
In his commentary on Sanhedrin, the Ran wrote in the 14th century CE that like the Christians, the Muslims by worshiping their religious objects were engaging in “divine worship” of idols, and therefore “their (i.e., the Muslim’s) ruling is of idol worship.” Incidentally, the Ran also referred to Muhammad as “the crazy person of the Ishmaelites.”
In his classic Torah commentary on Deuteronomy 4:28, addressing idols of “wood and stone,” the Rashbatz (14th-15th century CE) .[christianity and islam being idolatry]. he also spoke about the phenomenon of self-censorship among Jewish scholars on this topic, stemming from fear of persecution under Muslim rule
Rabbi Yosef Karo (15th-16th century CE), author of the renowned Jewish legal work The Shulchan Aruch, weighed in on Islam in his mystical book Maggid Meisharim. There he spoke of entering a Sufi Muslim house of worship, and later the same night being told by an angelic being that this constituted “turning to the idols and going after the Baalim.”
In a modern ruling, former Hebron and Kiryat Arba Chief Rabbi Dov Lior noted that in rejecting the sale of land in Israel to Muslims for Heter Mechira, the Hazon Ish referenced the Radbaz’s ruling that Islam is considered idolatry and one must die rather than convert. In his review of the issue, Rabbi Lior clearly showed the ban on selling land in Israel to a non-Jew does indeed apply to Muslims.
Rabbi Uri Sherki, director of the Brit Olam Noahide World Center, [makes an interesting point that muslims don't worship the full divinity, rather just the traits of judgment and strength, islam lacks personal group prayer as it lacks the concept of a compassionate God]
As for RAMBAM. While the Rambam did say that Islam was monotheist, and that their three practices of idol worship were now disconnected from idolatry, he also wrote in his legal responses (Tshuva 269) that Muslims were “not far from idol worship.” Elsewhere (Mishneh Torah, Halachot Tshuva Chapter 3), he clearly stated that the Muslims “deny the Torah.” This stands at clear odds with claims that Islam worships the same G-d as Judaism, and indicates the Rambam’s position was not as clear-cut as many present it. [RAMBAM also stated in his responsa, that it's forbidden to teach muslims Torah]
Another rabbi not mentioned in that article, the Tzitz Eliezer , considered Islam as idolatry. see Jews allowed to pray and join Muslim congregation in Mosques and Experiences