Thank you for posting a more concise description of what this thing is really made out of. The link @gortex2 posted wasn't as descriptive and had me assuming, and you know what they say about assuming, that it is that fancy new marketing speak that's so prevalent these days. They didn't even mention plating. This reminds me of going to Home Depot to buy a garden hose with "copper infused" fittings, which simply means anodized aluminum that is colored to look like copper or brass.
And yes, I do know what these differences are when it comes to metallurgy. I'm just not proficient posting from a smartphone. Yes, brass is made by alloying copper and zinc. Also, pure zinc or magnesium is used in sacrificial anodes in the corrosion prevention sector. And now that I see that they are using "zinc alloy" which indicates it's not pure zinc, not that it matters in this application. The primary reason these NMO mounts are being manufactured with "zinc" is it's much cheaper than brass, as you indicated. And requires very little machining, if any, as these mounts are die-cast. In my opinion, these are equivalent to Chinese pot metal.
That said, I'm confident these mounts are still going to be quality from an electrical and mechanical perspective and one shouldn't have any second thoughts of using one. Call me old fashioned as I've been using quality brass NMO mounting kits for close to 40 years. Plus, I have a nice stockpile of kits to last me two lifetimes. No zinc for me.