I'll probably get hate mail, but I think he's right to a certain degree. Marx was first and foremost a believer in dialectical materialism. Dialectical materialism states that there is a natural progression of the economic mode of production. He was thus pretty certain that capitalism was better than any economic system that came before it. He simply theorized about how the next economic mode of production might be attained and what it looked like.
So yeah. Marx was indeed a capitalist.
As for the book he wrote about capitalism? I'm not sure about you, but a division between the owners of the means of production and everyone else where the one owning the means of production is disproportionately advantaged compared to anyone else doesn't seem like a positive assessment of capitalism. Hell, he even thought it would inevitably end because it was an extremely unstable system, but if that's further proof that Marx was a capitalist then be my guest.
I admit I'm disappointed.