"That's just rose-colored speculation, right?"
Considering that Dr. Edwin Kilbourne is an emeritus professor of immunology at New York Medical College I would put a bit more credibility into his assessment than a random poster on FR.
"In any event, it hasn't much mitigated the effect on those thus far infected."
The point he's made, and apparently your missing, is that thousands may have already been infected with mild symptoms. If true, it paints a much different picture than what is being currently portrayed. Observational data from Turkey would certainly suggest a lower mortality rate than what the UN has advertised and it suggests that Kilbourne's hypothesis might be credible.
Many observers have speculated such. Little hard evidence has been presented that this is the case. If you are in possession of the smoking gun I know numerous immunologists, epidemiologists, and virologists would love to see it. The point you, and apparently he, are missing, is that widespread human infections would be a bad thing, even mild ones, as many additional opportunities for recombination and, especially, reassortment would occur.
It appears that this Ed Kilbourne has his posterior glued to his rocking chair and doesn't venture out in the field much these days. A brief Goggle on his name and H5N1 will show that many have already dismissed his comments, such as this poster:
It seems to me that Dr. Kilbourne makes several statements based on old paradigms. Yet a lot of the public are going to accord him expert status on the basis of being an emeritus professor.
Would you be included in that group?