Identifying conflicts of interest when evaluating systems is mentioned in the ACM Code of Ethics. It's possible to do this in a neutral/professional way.
2.5 refers to 1.3, which is clearly about identifying one's own possible conflicts of interest. There is no suggestion that one should accuse others of having such conflicts. Even if such an accusation were to be made, it should be specific and neutrally phrased, not a passive-aggressive blanket condemnation of anyone who might disagree.
The only way that applies, even considering that IETF and ACM are two separate organizations, is to Fielding's own conflicts of interest.
Point is, it's relevant to technical discussions like this. Also Section 2.5 doesn't really limit itself to one's own conflicts of interest (only by reference to the other section). But that's beside that point anyhow I think, as either way it rebuts your point that "Attribution of motive is never helpful or appropriate as part of a supposedly-technical discussion".
Yes, ACM doesn't really mean anything for IETF, but ACM and IEEE are (pretty much) the only two professional associations for us.
The only way that applies, even considering that IETF and ACM are two separate organizations, is to Fielding's own conflicts of interest.