I already knew that.
Even under the wider discipline of political economy Marx wasn't the most important. For actual analysis of the interaction between the economy and politics, arguably Carl Menger is far more important because his method that emerged out of the Methodenstreit is far more philosophically coherent than the Hegalian nonsense that Marx bought into.
Hegel was Marx's starting point in academia. It was far from his end point. By the time
Capital was published, Marx had totally abandoned any pretense to Hegelian dialectics. Marx's own philosophy of natural science shares much more in common with Adam Smith or Aristotle than with Hegel. In effect, much of Marx's academic career has been one of excising Hegel
et al.'s idealism from philosophy and natural sciences.
Like I said, while Kapital had its moments, it wasn't all that important in the grand scheme of things. Only someone with an existing ideological proclivity towards it could say Marx was '...the single most important thinker in political economy of the entire 19th century'.
On the contrary, only someone whose ideological proclivities dictated a complete avoidance of anything related to Marx would be able to claim otherwise. Don't shoot the messenger simply because he is associating with an ideology you abhor. Like it or not, the social sciences as a whole, and particularly political economy, are tremendously indebted to Marx and derivative thinkers.