As far as I know, the MIA cites it as opposing Guesde on the matter of the economic section of the program for the French Workers' Party (the section was written by Guesde, and mainly, though not entirely, approved of by Marx). However, as far as I know, the quote is only found quoted by Engels, and I can only find it in two documents:
"Now what is known as ‘Marxism’ in France is, indeed, an altogether peculiar product — so much so that Marx once said to Lafargue: ‘Ce qu'il y a de certain c'est que moi, je ne suis pas Marxiste.’"
- Letter to Bernstein, 1882.
"And if this man has not yet discovered that while the material mode of existence is the primum agens [primary agent, prime cause] this does not preclude the ideological spheres from reacting upon it in their turn, though with a secondary effect, he cannot possibly have understood the subject he is writing about. However, as I said, all this is secondhand and little Moritz is a dangerous friend. The materialist conception of history has a lot of them nowadays, to whom it serves as an excuse for not studying history. Just as Marx used to say, commenting on the French "Marxists" of the late 70s: "All I know is that I am not a Marxist.""
-Letter to Schmidt, 1890.
Engels also references it here:
"We have never called you anything but ‘the so-called Marxists’ and I would not know how else to describe you. Should you have some other, equally succinct name, let us know and we shall duly and gladly apply it to you."
-Letter to Lafargue, 1889.
The second seems to imply that the reason for the quote was on people using the materialist theory of history as an excuse not to study history, rather than as a tool for doing so. It's possible that Engels referred to it in some letters not in the MIA, but I really don't see any reason to see a relation between the quote and Guesde, certainly specific ideas of Guesde (I suppose that Guesde could have been involved as a member of the group referred to). An alternative interpretation by Rubel reads too much into the quote, and generally seems to twist the quote into justifying Rubel rather than examining the actual context, it would seem, but nobody's perfect. That is, it would appear that Marx was merely disassociating himself with some French communists who called themselves 'Marxists', rather than anything more interesting. Still, I am curious as to where the interpretation involving Guesde comes from, and the conflict between the two in general. Marx did apparently refer to him as 'Bakuninist', but, rather than this having to do with his position on the economic section, it was probably on the 'general strike', with Engels commenting, "Paul spoke very well — a slight indication of the universal strike dream in it, which nonsense Guesde has retained from his anarchist days — (whenever we are in a position to try the universal strike, we shall be able to get what we want for the mere asking for it, without the roundabout way of the universal strike)." So yeah, I'm just curious as to the historical accuracy of the various claims relating to the significance of the quote. and interested on if anybody can find anything else relevant to the context of the quote. The previous thread on the quote doesn't leave me feeling particularly encouraged, but anyways. Also, if the MIA's reason is true, I'm not sure one could call it an attack on Trot-style 'transitional programs', seeing as Guesde wasn't, it would seem, actually advocating an 'unattainable' (well, supposed to be) program.
Also, just to clarify, the 'political' section of that program was the 'maximum program', while the economic demands were the 'minimum program'. The debate was, I believe, over whether the latter were plausible under capitalism. Anyways, also perhaps of interest, it seems that Engels said that the label of 'Marxism' was made by their opponents (presumably as an insult, like 'De Leonite'). To be honest, it does make debates on whether the label 'Marxism' involves worship of an individual seem rather absurd, or more absurd than originally, rather, seeing as it was meant to suggest that as an insult, and then presumably adopted as a counter to said insult (or to nullify its use as an insult and annoy the people using it as one, I suppose). But anyways, while it may have some relevance to Rubel, it's mainly off-topic, so never mind.