While early socialist writers got a lot of things right, they also failed to overcome a lot of social prejudices. Engels, for example is documentedly a homophobe and there's no documented evidence (to my knowledge) that Marx ever saw a problem with or pushed back against that. Marx denied his daughter permission to marry the man she wanted to marry and there's no evidence Engel's pushed back against or saw a problem with that. Despite coining the maxim "from each according to their ability, to each according to their need" they were probably both fairly ableist. Modern communism has moved past Marx and Engel's latent sexism (which they to their credit did write about) and blatant homophobia. I hope we can move past their ableism as well.

Calling your comrades intelligence based slurs or thinking of them as lesser because they were born less intelligent than people around them isn't self criticism it's bigotry. Calling reactionaries ableist slurs is also bigotry. If there are flaws in your comrades ideology or reasoning then pointing out those flaws is doing them a service. If reactionaries haven't thought through their arguments or are being evil fuckers then absolutely point out those flaws and tell the world they're scumfuck shitlords who deserve to die screaming for their sins. You don't need to use bigoted slurs to do either of these things.

On "there will always be some people born smarter/stronger/etc and they will be more powerful than others because of this" yes, but it is our responsibility not to socially exacerbate this. To the greatest extent that we can shouldn't make people feel more or less valued or accepted because they learn, build muscle or make friends faster and we certainly shouldn't award people more commodities because of these traits. We shouldn't treat people's ideas as more or less valid because we think that person is less intelligent and we shouldn't respond to ideas we think are bad by saying "I think the person who came up with this is less good at thinking than other people".

When you call someone stupid or dumb for saying a certain thing, it probably won't have that much effect on them if they don't get called that outside of the context of internet arguments. But if they spent most of their childhood feeling stupid or being told they can't try things because they're mentally incapable then using insults/slurs based on them being below average intelligence is in effect a social bludgeon attempting to shame them out of the discussion for being incapable of understanding it. Aside from being a really shitty thing to do to someone mentally/emotionally in the context of trying to do self crit/ideological recruitment this has the effect of saying to them "you're too unintelligent to understand communism so you'll just have to do what I the smart person who understands communism says" and overall creates the impresssion that communism is for smart people only. Which is very damaging especially in the capitalist anglosphere where a lot of the working class have been told that the reason they're poor is because they were too stupid to get a real job.