Its not a lie, guppies are disgustingly over bred and most likely incredibly inbred.
Sorry but if anyone knows anything about breeding for specific colours then they know this will be true. If you breed a beautiful male pure yellow guppy to a averagely nice female, you keep back the best quality pure yellow female babies to put back to the same male (their father) to produce more intense yellow babies.
Same with Black Mollies etc or any recessive colour, to keep/improve the colour, it will involve breeding back at some point and with the rapid breeding livebearers, it will happen far too regularly. Especially bred in huge vats and ponds, nothing to control who or what is breeding with whom.
That aside... bright red gills generally means one of two things, less likely... gill flukes/parasites... or more likely... ammonia poisoning.
Actually.. thirdly... seeing as lots of fish are so weird looking these days, they might have transparent gill plates or have a genetic defmormity meaning they dont have gill plates!