By workers he clearly means labor unions. Individual people who are not engaged in the revolutionary struggle or the defense of communism should not have access to guns (or any weapon for that matter). Western leftists, particularly American ones, have liberal brain worms because they intertwined this with the second ammendment. Not everyone who is a worker should be entrusted with a fucking gun, the organizations that represent workers however should. Its the same argument with cars, are cars bad? Not inherently, but we as a society should be phasing them out. Western leftists listen to China challenge (HARD)
No he didn't just mean labor unions: https://www.marxists.org/archive/marx/works/1847/communist-league/1850-ad1.htm
Everyone should read the whole thing, but here's an examplary excerpt: "The whole proletariat must be armed at once with muskets, rifles, cannon and ammunition, and the revival of the old-style citizens’ militia, directed against the workers, must be opposed."
Marx is essentially saying that the communists need to organize and arm the proletariat ASAP and that bourgeois forces are already effectively pushing back - arming is necessary. Marx isn't talking idealistically, he's talking specifically about the need to forcefully oppose reaction to communists openly organizing against capital rather than secretly.
It's not difficult to make the case that socialists nowadays also desperately need both organization and arms. As an example, I recommend that everyone read The Jakarta Method, paying specifically close attention to the section asking survivors what they wish they'd done instead. The violence will come to us as we become a threat (before that, actually), so we need to protect ourselves and our comrades.