It generally is, and she certainly believed it to be so & behaved accordingly.
Let's consider what it is to be a "dynastic line:" it is the unbroken rule of a successive line of sovereigns from the same family or clan. One ends when it fails - either through lack of heirs or, more commonly, when it is overthrown and replaced by another line with enough force (or guile) to do so.
Wu achieved that. She easily outlasted her first two husbands (though calling Taizong her "husband" might be a bit of a stretch, as she was little more than a serving girl of the Inner Court during her time there in his reign). Then when Gaozong finally bought the farm, she dutifully placed first one, and then another of her sons on the throne in her role as longtime co-ruler & Dowager Empress... before deciding, one after the next, that neither was worthy or up to the challenge of the position, and she'd be better off doing it herself.
Thus, she forcibly deposed her 22 year old son Ruizong, donned the Yellow, sat the Dragon Throne, and proclaimed herself the Huangdi of a new regime - the Zhou Reborn. This act effectively ended the reign of the Imperial House of Li, which was replaced with the Imperial House of Wu. True to form, she enshrined seven generations of her Wu ancestors within the imperial ancestral temple. She established The-Artist-Formerly-Known-as-Emperor-Ruizong-of-Tang (Li Dan) as her heir apparent, but with the stipulation she she adopt her royal surname, thus becoming Crown Prince Wu Dan. She was also sorely tempted to replace her son with a nephew of hers that bore the surname Wu "in truth" and by birth... though she ultimately decided against a further ouster of her already humiliated younger son. Instead, she decided to render her nephew - Wu Chengsi - as her Chancellor.
By every measure, she had established a new dynastic order in fact, and then maintained it for 15 years.
Now, granted, her new Zhou Dynasty did not survive her own eventual overthrow at the hands of her eldest son, the likewise overthrown-by-mom Li Xian, AKA Emperor Zhongzong, who managed to cobble together a pretty tenuous alliance of factions who agreed on little else other than "that 90 year old can't be allowed to embarrass us with her uterus any longer."
Assured by his comrades that he did have enough backbone to stick up to his other for once in his life, and backed by 500 Yulin Palace Guardsmen, Zhongzong's force entered the imperial city through the Xuanwu Gate, and then turned to the inner gates of the palace, making to enter and… do what needed to be done.
But instead they found barring their way to the inner gate, a disheveled but wrathful-as-all-hell Empress Wu. She’d seen what was going on and understood what it meant completely. Nevertheless, she approached the column of soldiers arrayed against her, marching to her trembling son Zhongzong and the leader of this apparent coup d’etat. Though she tinged her words with contempt for this move against her, she was willing enough to yield the throne to her heir, Li Xian, Emperor Zhongzong. Then, her time as the Empress Regnant of China officially at its end… this shrunken, frail, sick, 90-year-old woman went back to bed.
She would be moved under heavy guard the following day to a smaller but nevertheless still imperial palace, and on March 3rd the Zhou Dynasty was ended with the official restoration of the Tang. Empress Wu would write a final edict before her death later that year. In it, she should no longer be referred to as huangdi, or Emperor, but rather as Zetian Dasheng Huanghou, The Holy Empress Zetian. Then, she died… peacefull on December 16th, 705, at between 90 and 92 years old, having lived in the imperial court for more than 75 years, been Empress of China for 35 Years, and reigned supreme as Divine Sovereign for 15 years.
The Tang was restored.
If the argument against her reign being an actual Dynastic Order hinges on the fact that it was a dynasty with but a single ruler - well, that actually throws a fair few of other short-lived Dynasties into question. Wang Mang's Xin Dynasty that split the Han in half didn't survive his fiery fall... would he therefore be removed from recognition, as well? Qin barely managed to survive the death of its First Emperor, only to be quickly squandered by its Second and overthrown... all in the span of that same decade-and-a-half that Wu Zetian commanded All Under Heaven in her own name and right... yet you'd be hard-pressed to find anyone willing to seriously consider the Qin to be not a "true" dynastic order.
The argument again Wu Zetian's Zhou Dynasty not being considered "legitimate" has historically hinged almost completely on the fact that it was established and ruled over by a woman - and nevermind that she just so happened to be very possibly the most brilliant mind of the age... the fact that she dared overstep her position as a mere female to take command of men and nations was an abberation and an insult against the "natural order" that the Confucian scolds across time were never willing to accept, allow, or even admit to.
Clements, Johnathan. Wu: the Chinese Empress Who Schemed, Seduced and Murdered Her Way to Become A Living God.
Dash, Mike. “The Demonization of Empress Wu” in The Smithsonian
Fitzgerald, C.P. The Empress Wu.
Guisso, Richard W. L. “The Reigns of the empress Wu, Chung-tsung and Jui-tsung (684-712)” in The Cambridge History of China, vol. 3.
Jiang, Chen An. Empress of China: Wu Ze Tian.
Also, if anyone's so inclined, here's my own 8-part series on her life and times: Daughter of Heaven: The Wu Zetian Suite