I am not Marxian nor Marxist in anyways so my views would not be reflective of these sorts of views. That being said... the context of this quote and of the author is important.

It is often asserted that while Marx's theory transcends bourgeois economic theory in order to solve "economic problems" that cannot be satisfactorily dealt with by bourgeois price theory, it must, for that reason, be as empirical as any other science. It is assumed, in brief, that Marx's Capital is a better part, but still a part, of the "positive science" of economics, whereas it is actually its opposition. Marxian theory aims not to resolve "economic problems" of bourgeois society but to show them to be unsolvable. Marx was a socialist, not an economist. In Marxian theory the concrete phenomena of bourgeois society are something other than they appear to be. Empirically discovered facts have first to be freed of their fetishistic connotations before they reveal empirical reality. The abstract generalizations of value theory disclose the laws of development of a system that operates with a false comprehension of the concretely given facts. The inductively won data do not correspond with, but camouflage, the real social relations of production. Bourgeois economy is not an empirical science but an ideological substitute for such a science; a pseudo-science, despite its scientific methodology.

In other words: the author argues that despite the perception that Marx's work Das Kapital was a work of economic theory (compared to for instance Smith, Ricardo, Keynes, and others), Marxist thought rejects outright the constraints of economics. In effect the author argues that economics is a false discipline with no basis in reality. Incidentally, Paul Mattick Jr, the son of the more famous Paul Mattick Sr, is the one that made this quote.

Essentially, modern economics relies on two key ideas: a) people in aggregate are rational (meaning that they will seek (in the aggregate) to pursue their best interests) and b) that we can use math and statistical data to help model these out. Mattick Jr. rejects the use of math as he argues that the assumptions that economics rely on are false and are a construct of our "money-based" society.

Now whether or not his allegations have any actual basis behind them is uncertain, but suffice to say that this was published in 1983 and economics is still going strong. If anything, the discipline has become even more quantitative. Make of that what you will.