JSON is a subset of javascript which already existed. He just coined the phrases 'json' and wrote it down.
First, because some people will treat that authorship credit as a warning label rather than authority; that would make it ad hominem, not appeal to authority.
Second, because it actually gives useful context to some parts of this spec: "Oh, it makes sense that the author of a language without integer types would propose a spec that claims you don't need integer types". That's not a logical fallacy at all; that's perfectly reasonable reasoning.
I don't think the person you replied to was claiming that DEC64 was good or bad because it was written by Crockford? Didn't feel like he was passing any judgement, merely pointing out who the author was...
Also, see: http://www.logicalfallacies.info/relevance/fallacists/