That is clearly not the intention, else there would be no reason to bother with font stacks. The intention is to list a set of ranked preferences, implemented as fallbacks, that best express the designer’s vision for the site, while avoiding font downloading. In your example, presumably the designer thinks that Consolas works better than JetBrains Mono NL.
Yes, I see what you mean, but if that's really the intent, then having only this font-family rule isn't sufficient. For that argument/reasoning to hold up, it would have to be accompanied by metrics adjustments to compensate for the massive differences between some of the listed fonts in these stacks.
Absolutely true. In fact (and thank you for getting me to think about this), font stacks without metric adjustments don’t really make much sense, do they? (Unless confined to a list of fonts that are very close metrically, which they never are.)