But what does the ruling ideology and its effects on the definitions of words matter on a leftist sub in a discussion between two leftists?

Because we are still using language, specifically English, that is affected by capital. No manner of communication escapes capital's affect. It is pervasive.

If you try to create arguments for leftist positions using rightist language, you'll never end up sounding like anything more than a left liberal.

Well that is not want I meant. I am saying that we have to be aware of how capital affects our language and argue accordingly. This does not necessitate using rightest language, merely that we have to understand that people from the right will see our language differently than we do. In other words, if you were to say, "Owners don't produce value!" to a capitalist they will probably look at you funny and go "Of course they do, they make tons of money!" This is because both of you are arguing from completely different definitions of value. The worst response in this case would be to ridicule or be sarcastic to the person. Rather you would need to actually discuss how and why you think value should be defined as you do. You see, both of you are making a claim to knowledge. The power in this case rests in the person that is assumed to hold knowledge and when both claim knowledge the tendency is to attempt to reclaim power by trying to discredit the other person's claim to knowledge either through appeal to "facts", careful argument for their point, or the old standby of ridicule.

Differing meanings is one of the most common points of disagreement, but since there is no objective ground for meaning it is incredibly difficult to actually argue for any one meaning. This is why people hate arguing over semantics while, at the same time, it happens so often. It is hard to gain power in discourse when you actually have to determine why you define things the way you do since most people never give any thought to it. Without giving any thought as to why you accept one definition over another you have to resort to other methods of gaining power, all of which can affect the outcome of a debate dramatically and often for the worst.

That is why I am talking about the definitions of word in a leftist forum.

It's important to argue from the left rather than from the right.

But the left-right binary is itself a product of capital and is affected by it just like everything else. Just "arguing from the left" is not in anyway escaping the confines of capital. Just look at America where morons like Bill Maher are considered "far-left". What is determined as left and right differs widely between semiotic spheres. One must understand how the left is viewed to be able to effectively argue against the right. This does not mean that we shouldn't argue from the left, just that we must do so with proper context.