It honestly depends on how you want to define it and under what framework. In Marxism the definition of class depends on the relationship to the means of production. Obviously using the Marxist framework is the most accurate to describe what’s actually happening and as Marxists we should use those definitions and try to inform people as best we can that proletariat and bourgeoisie are the two main and oppositional classes (primary contradiction) in capitalism. But like Mardoniush pointed out there still are other classes since the relationship to the means of production can be more complicated than just straight ownership vs labor despite the antagonism of those two classes being the primary contradiction.

Still, none of those are a “middle class” and you are completely right about how one of the big reasons that term gets heavily used is so the parts of the proletariat that are not impoverished can feel elevated above those who are. But that right there means there is a material difference between certain subsets of the proletariat that gets called “middle class” and “lower class.” There is a large subset that historically in the US was generally well off (comparatively) to the subset that lives in poverty conditions, and it is true that the former subset is shrinking while the latter subset is growing (along with the intensity of the contradictions). It is not wrong to point this out, and “middle class” is definitely not meaningless when used this way, it’s just that using the word “class” to describe this phenomenon is deliberately muddying the waters and it’s not using the term according to a Marxist framework. Don’t mistake that to mean that what the OP article is saying is untrue or isn’t happening, because it most definitely is happening! It’s just bad semantics.