This isn't true. If you are talking about costume jewelry made in the last few years, even some branded designer pieces do not hold value. But there is a vast and valuable market of pieces some might describe as costume (including some sterling silver pieces) that are substantially more valuable than gold pieces by virtue of their design, rarity, age or a combination of those and other factors. I think there is a real sense of snobbism (not directing this at the OP, the person quoted, or anyone in particular!) that is something isn't a design visibly made by a particular jeweller, or a brand name, than it has no "value." My personal preference: I tend to shun most pieces that are very popular by virtue of the fact that I would rather express my style in a more individual way. Antique and vintage pieces allow me to do that more easily.
And when it comes to all jewelry, value is very subjective and can be affected by a lot more than the materials involved. I go to a lot of antique shows and markets and tables are flooded with really mediocre gold pieces that may hold some value based on gold weight. Of course, the same can be said of costume pieces - lots with little design or other merit. But to class any type of accessory as "worthless" is a pretty broad value judgment that doesn't hold water.