Yeah, but the same example that you sighted is an example of of the "loss of genetic information" in both populations, and they are (neither of them) significantly different from the original hawthorn fly; there is no new species because there hasn't really been a change in kind (and additional genetic information), only a loss, only a difference.
Well, I suppose it is if you pull a couple of quotation marks out of nowhere and simply assert that it's only a "loss of genetic information". Of course that doesn't reflect that there is merely a difference in alleles, a difference in feeding patterns, a difference in growth and seasonality, etc.
There's nothing in what 49th posted to tell you that this is due to a "loss of genetic information" in either population, let alone in both. Indeed, at least between both populations collectively, there has clearly been a gain in genetic information simply from the fact that they have different alleles (different versions of some of the same genes, which means more versions in total).
Besides, there's no rule that evolution can't occur from the loss of genetic information. It can occur that way, and/or from a gain of information, and/or from the content of genetic information merely changing. ALL of those things (if they become fixed in actual populations of living organisms) are evolution by definition and by fact.