I think he just meant you shouldn't go to standard dictionaries expecting to find speculation on Saitama people or reliable information as to its origin. I've also found dasai online and in print, although even if it were left out that wouldn't necessarily be any implication of its valence of course.
At any rate, this all came up in a discussion of the negative and positive associations of the word "Ikeru" as seen in the title of the 2003 book 'Iketenai-Nippon Nihonjin-No Honto-No Tokoro' and why the standard translation of 'Why Japan nerves' is actually much more offensive and damning in its native Japanese (written in Japanese by a German national living there I believe) than English.
So here's the first part:
イケるとかイケてるという表現はかなり前からありました。
意味は"格好良い"や"おいしい"や"美しい"というような肯定的ことですが
、英語のCOOLみたいな感じだと思います。
イケてないは、最近の表現で"格好悪"いとか"まずい"とか否定的な
意味になります。英語でなんていうかよくわかりません。
本当のところはTRUTH OF ~でいいと思います。
ださい日本-日本人の真実といったところでしょうか?