I agree. It's rather worse than that, actually. It's an aggressive insistence on looking at taxes in total isolation. And the whole thing has a very "gotcha" quality to it. The media loves to "gotcha" candidates, but in this case they are doing so by playing off the public's obsession with paying taxes to the exclusion of all else. It isn't a very informative way to approach the subject.
Still, Warren could have given a more candid answer. She should have said, "yes, middle classes taxes will go up somewhat, but the savings on health care will be far greater than whatever you'll pay in additional taxes. My proposal will make the average middle class tax payer better off financially."
The reason Warren doesn't want to answer the question is that if pressed, she has to admit that the above will not be true for everyone. With Medicare-For-All, those who use more medical care will do way better. My family is in that boat. We had to spend almost 10K last year on healthcare because of yearly deductibles and none of us has what I'd call serious health problems. But those who use healthcare barely at all, like people in their 20's, will likely pay more in taxes than they save on healthcare. Warren is ducking the question of taxes because she doesn't want to admit that.