I have to agree with your line of thinking.
While Tagless is trying to pin Faith to a wall with his nitpicking of what she said, he is giving the impression that the DSS offers no support for her position.
She says that the similarity between the DSS manuscripts and the modern versions of the Bible give a strong indication that the majority of scribes, through the centries, maintained high quality control standards in their copying of these texts. This care in maintaining the text, gives some credence to the supposition that the scribes, or their supervisors, held these documents in high regard...even as sacred. This high level of belief by those in charge of the records speaks highly of the veracity of the content of those documents.
Tagless cannot say that the Isaiah scroll is not evidence in that regard. Even if it is not the exact document that was copied, it is still evidence that the document that
was copied was the same as it or very similar. If the original document from that time, copied down through the centries was changed, purposefully or not, the chances are small that it would have turned out so similar to the DSS Isaiah, if it was not already very similar in the first place.
Questions that might be asked are:
Was the quality control more stringently adhered to for Biblical texts than for non-Biblical texts?
Does the strictness of adherence to quality control relate in any way to the actual truth of the documents copied or is that strictness a product of the belief that they are true?