I wouldn't say I felt "insulted" by the article or that particular sentence - it's just not what I would have expected. The information still helps to convert at least the races - however, it's more of a "designer's notes" than a conversion document. Designer's notes as such are fine, and they're actually often quite helpful, even in this case (you can easily convert your races after reading the article). My main gripe is that it's not "How to Convert your PCs" but "What We Did in Rewriting the Rules".
Then again, there are no easy answers for some of the conversion problems, regardless of how brilliant any conversion notes may be. If you have characters above level 10, for instance, the only solution is to either pick a class combination that best approximates the PC's abilities, or extend the classes as presented beyond level 10 - the progressions are all fairly obvious. I doubt, though, that Mongoose will actually give you "extended classes" for conversion - had they wanted to do these, they would have been in the rules in the first place.
A few hints, however, which class combinations might be useful to best approximate single-class levels above 10... now that would probably be nice.