I also don't believe that the article that Vinny attached proves the premise that "most Malawi Cichlids in the hobby are hybrids."
That said, it makes sense for all of us to be skeptical of the "purity" of cichlids in the marketplace, farmed, "imported" or from hobbyists. Without demonstrable provenance to the wild, it's best to assume that most cichlids aren't pure (or at least pure enough for inclusion in, for example, breeding projects with known provenance fish).
Apart from crooks selling pond or farm-raised fish as "wild" (hey, that's what I was told they were!), poor husbandry practices can result in inadvertent hybridization. For example, breeding multiple mbuna, Hap or peacock species in the same tank or pond can result in crosses.
This article (and it was a series of back-and-forths, if memory holds) was about the explosion of albino peacocks, haps, etc. on the market at the time. It seemed quite fishy at the time and still does (at least to me).
The bottom line is that 99+% of the aquarium fish market values new, colorful and cheap over verifiable purity. That a whatever peacock is 1/64th some other kind of peacock in order to carry albino trait couldn't matter less to most. Or that a whatever peacock was developed by crossing two different geographical varients of the same peacock species (or two different peacock species).
We're the 0.5% who might care. But be prepared to pay a premium for it...and take the care to practice proper husbandry and labeling of fish that you breed and sell!
Matt