In the "Action Philosphers" comic book series, at one point in the Karl Marx chapter there's a joke at the expense of a film professor deconstructing marxism in movies (or something along those lines, this is from memory), and their version of Marx says the above line. Normally I would just brush that off as a joke, but this came with a footnote claiming that is an actual quote (without providing context). So I looked it up online, but I just got a confirmation that the quote is real, and that it boils down to distancing himself from the various interpretations of Marxism during his lifetime.
Do we know in greater detail what he distanced himself from, and what his own views were instead?
Hey there!
Maybe I can help. I am a political philosopher but hopefully I can provide something up to standard and something that will answer your question adequately.
I know which part you refer to in the comic book. So in reference to the comic book, it refers specifically to those intellectuals (such as the film professor) not ready to do anything meaningful for the workers. In other words, marxism required action and stirring awareness in the masses was often part of what the intellectuals were expected to do but the ivory-tower stuff such as "marxism in film noire" (if I remember correctly) were doing nothing but allowing the bourgeoisie to be in place or even be part of the bourgeoisie. That is, at least, what there is to be understood in the section you refer to. If being a marxist is doing what this professor is doing then Marx is not a marxist.
But the quote goes further. The context of the quote is circa 1880 when Marx assisted French intellectuals in writing the program for the Parti Ouvrier de France (worker's party of France). The Parti Ouvrier wanted a program and Marx helped Jules Guesde write said program.
Although parts of it were good as far as Marx was concerned, he had issues with demands such as limited number of work days and better work conditions. For Marx, if those demands were what being a Marxist is, then he was "not a marxist".
Now is the interesting part (in my opinion!): why supporting those things would make Marx not want to be a marxist?
Well, for Marx those were tools to make workers become less radicalised. It is a process that would gradually incorporate workers into the Bourgeoisie and make the alienation comfortable. Even in the communist manifesto, it is something that Marx warns his readers about: one of the most dangerous Bourgeois is the one who will propose free healthcare and other social benefits because the workers will be appeased, not realising their condition and their alienation and instead work within the confine of the system. Workers will not emancipate because the system will absorb them into it.
In a way that might sound familiar today (at the risk of oversimplifying it): by giving people better salary, conditions and less work while allowing them to drown themselves in superficial consumerism, it would allow the Bourgeoisie to keep the reins of power; i.e. control the means of production.
Does it mean that Marx wanted to keep the workers miserable? Not really but he believed that the way to go about it was to have the proletariat control the means of production and better life conditions would follow and we had to leverage the condition of the workers to emancipate them and not lead them directly into apathy.
This became a central topic later on: following World War II, the Frankfurt School were a bunch of marxist intellectuals and were puzzled as of why, during the 1930s, the proleteriat did not rise up as they expected it to. The economy was bad, overall society was not doing well from the worker's perspective and fascism was able to take root. So it is definitely a recurrent issue in Marxism! How come are people not overthrowing the Bourgeoisie? There was hope that with economic collapse, the workers would radicalise quickly after (and some did radicalise) but overall it did not occur. A lot of historians would probably tell me there were significant marxist movements in many places (such as France) and we saw many groups rise up following WWII but nothing that would qualify as an empowering of the proletariat on a large scale.
I would like to finish on one note. In unions, the whole issue was a problem. Per example, in Québec (where I am from), unions were at a time confronted to the same problem as Marx. Many union members wanted a militant worker base that would seek to free the proletariat and allow workers to leave their alienated state. Good working conditions were seen as clouding the movement and making their alienation comfortable. It had a significant impact and created turmoil within unions.
In Québec, I know that we referred to the unions seeking "better conditions" over an emancipation as "American-style". I do not know enough about American syndicalism to know if its warranted but it does give the tone that eventually, unions became a lot more about benefits and ensuring a decent salary than about emancipating the people from the bosses. Although breaking the 20 years rules, I think one can look at most unions today and see that it has come to be a lot more about being a counterbalance to managers and ensuring workers protection than about taking control of the means of production.
I hope it was clear! English is not my first language :) If you have any questions or if something is unclear, do not hesitate!