I disagree that the original response was not an argument. I'd ask that you review Anderson's letter again. I'm also unsure why you are attempting to connect epideictic rhetoric (used in several places in the letter) to the logical flaw in question since the rhetoric you are raising occurs after the flaw in paragraph.
http://www.cl.cam.ac.uk/~rja14/Papers/ukca.pdf
It's presented as an argument, it lists flaws in the UK Card Association's reasoning asking for censorship of the research. It presents a clear conclusion ~"we will not censor the thesis" supported by clearly labeled premises, specifically:
1) The Card Association incorrectly addressed the complaint so they are asking the wrong people to censor the thesis
2) Because powerful groups are against a research area is not reason to censor the work. I can see how one might find this premise confusing since he only dwells on it for the first sentence of the paragraph. The rest of the paragraph is composed of the logical flaw in question, and then epideictic rhetoric about the noble business of Universities followed by an FU statement that they are also going to ensure the thesis spreads far and wide in direct opposition to the complaint. In other words, the second paragraph is a mess.
The argument of the premise is simple and is in fact an admonition against the logical flaw the complainant is using "we are strong so you must do as we say", but Anderson blends and conflates it with rhetoric and the flaw, concluding "thus we have no choice but to back him, that would hold even if we did not agree with the material!"
3) The thesis is not new information anyways, so to ask for its censor makes no sense.
4) The source was not made available, so to the consumer of the thesis, they only have knowledge that the vulnerability exists, ergo they are asking to censor something that does not harm
5) The test of the thesis was not fraud, as the complainants claim, therefore asking for censorship under that claim makes no sense.
6) That the thesis exposes a vulnerability that undermines public confidence in the security of the system, but since the flaw has been apparently fixed, there is no reason to censor the thesis.
Conclusion) We will not censor the thesis because the logic of the premises hold true that the thesis should not be censored.
I do agree with you that the original response is not a formal logical argument in the strictest sense, and has elements of rhetoric (particularly in the 2nd premise), but it's clearly framed as at the very least a rational argument.
The letter in it's entirety are certainly not epideictic by any of my understandings of the term nor by your link. However, both the second and sixth premises contain elements of epideictic rhetoric. (btw, the presence of the logical flaw doesn't necessarily invalidate the 2nd premise since the passage is non-sequitur to the premise anyways!)
I think that's what you are trying to say here And not everything you say, even in the course of an argument, is itself argument. And I would agree with that sentiment. However, in this case, the sentence in question is clearly part of a logical argument and is a classic usage of a logical flaw.
I fully understand how Anderson would have written such a thing, impassioned as he must be over the issue. I actually found his argument persuasive and much milder in tone than I would have managed.