From the context of the comment it was pretty clear that they were using theft as shorthand for taking without permission.
If I have something, and you copy it, then I still have that thing.
The point of anti-copyright advocates are that merely copying is not ethically wrong. In fact, Why Software Must Be Free made the argument that preventing people from copying is ethically wrong because it limited the spread of culture and reuse.
That is the crux of the disagreement. You may rephrase our argument as "physical theft may be bad, but digital theft is not bad, and in fact preventing digital theft is in itself bad", but the argument does not change.
Of course, there is additional disagreement in the implied moral value of the word "theft". In that case I agree with you that pro-copyright/anti-AI advocates have made their point by the usage of that word. Of course, we disagree, but... it is what it is I suppose.