I checked again the Graham Robb biography (which is very good by the way, not just on Rimbaud the poet, but also on Rimbaud's second life as imperialist). He says the evidence is a bit murky, but still thinks there is enough to say that he spent some time in Paris in the days of the Commune. Perhaps more to the point, according to Robb, Rimbaud and Verlaine spent time in London, among the exiled communards like Vermersch and Lissagaray, going to meetings of the Cercle d'Etude Sociale in Compton Street, Soho, where, Robb thinks, he must have met Marx many times. That post festum Communard affiliation might be the factor that sealed Rimbaud's reputation, it seems to me. Robb, Rimbaud, 2000, pp 78, 189.