Now it just changed to the above, and Google has this thread index, whereupon it wasn't there a few minutes ago, as I have shown and everyone here would have experienced the same thing I did at the time.
Also from briefly reading a couple threads over @ the SMF forum on the issue, in this version of SMF, the search de-duplication doesn't work well.
I just noticed when I referenced a different thread from my watchlist (i.e. goto "new" posts) the noindex directive is given. This would support the theory that SMF sets it on "special" pages to avoid duplicate content for crawlers.
When I open the same thread directly from the main forum list, noindex is not in the source.
/end side-track.