if it seems that an argument does not contain all the information necessary to support its conclusions (because directly substituting in other words produces falsehood), this is because words have meanings, steps are elided, and there are things true and false in the world. This does not invalidate those arguments!

Thing is, this is a general response to virtually any criticism whatsoever. And it's often true! But it's not always a terribly useful response. Sometimes it's better to make explicit that bit of context, or that elided step.

Moreover it's also a good thing to remember about the other guy's argument next time you think his conclusions obviously do not follow from his (explicitly stated) premises - that is, next time you see what looks to you to be an invalid argument, it may not be even if strictly on a formal level it is, precisely because you are not necessarily seeing everything the other guy is seeing.

So, it's not just about substitutions. It's a general point.

These substitution arguments are quite a shortcut. The perpetrator doesn't actually have to construct something that supports a specific point; instead, they can take an argument they disagree with, swap some words around, leave out any words that are inconvenient, post it, and if the result doesn't make sense, the perpetrator wins!

Making a valid argument about why the substitution argument doesn't make sense requires more effort than creating the substitution argument, so if we regard discussions here as a war of attrition, the perpetrator wins even if you create a well-reasoned reply to him.

Substitution arguments are garbage. I wish I knew a clean way to get rid of them. Thanks for identifying them as a thing to be confronted.