how is karl marx a hegelian and how isn't he?

"The historicist insight is the final insight in the sense that it reveals all earlier thought as radically defective in the decisive respect and that there is no possibility of another legitimate change in the future which would render obsolete or as it were mediatise the historicist insight. As the absolute insight it must belong to the absolute moment in history. [...]

The absolute moment may be the absolute moment simply or the absolute moment of all previous history. That it is the absolute moment simply had been the contention of Hegel. His system of philosophy, the final philosophy, the perfect solution of all philosophic problems belongs to the moment when mankind has solved in principle its political problem by establishing the post-revolutionary state, the first state to recognize the equal dignity of every human being as such. This absolute peak of history, being the end of history, is at the same time the beginning of the final decline. In this respect Spengler has merely brought out the ultimate conclusion of Hegel's thought. No wonder therefore that almost everyone rebelled against Hegel. No one did this more effectively than Marx. Marx claimed to have laid bare with finality the mystery of all history, including the present and the imminent future, but also the outline of an order which was bound to come and in which and through which men would be able or compelled for the first time to lead truly human lives. More precisely, for Marx human history, so far from having been completed, has not even begun; what we call history is only the pre-history of humanity." (Source: Leo Strauss, _Studies in Platonic Political Philosophy_, page 32.)

In other words, while both Hegel and Marx consider the historicist insight the absolute insight which belongs to the absolute moment, whereas for Hegel the absolute moment was the absolute moment simply, for Marx it was the absolute moment of all previous history, or more precisely of all pre-history; it would not be followed by the final decline but, to the contrary, made possible or necessary the first real elevation, the emergence out of pre-history into actual history, out of the subhuman into the truly human.