I think its safe to say that "liberal arts" is way too huge of a category for comparison, and his assessment cross-contaminates the terms STEM and liberal arts. There are STEM majors that don't get you a BS.
I think if you polled STEM folks, the overwhelming majority would not categorize business and law degrees into the "useless liberal arts degree" bucket. In fact, how exactly do you exclude degrees like economics and business from the "M" in STEM?
His numbers also omit the fact that the majority of engineers earn those numbers with a bachelor's degree, whereas all of his "top earners" for non STEM degrees would require graduate degrees or doctorates.
Of course, the author is also under 40 and is a "social policy" director at Harvard, so he's even more biased than most of the people on this board.
BTW, if you look at MEDIAN incomes and not average, there is a swing in the favor of the STEM earnings of nearly 50% for mid-career earners...