It’s even worse than most people realize.
Basis weight is the weight, in pounds, of a ream of sheets at their original manufactured or “basic” size. While that sounds straightforward, different grades of paper have different basic sizes and, the number of sheets in a ream can also vary in some cases. So, 80lb writing stock and 80lb cover stock will be quite different.
It also doesn’t really provide any information on thickness (“caliper”) or rigidity/stiffness.
Grams per square meter (g/m^2) standardizes sheet sizes making the measurement comparable across grades of paper and, more intuitive.
I guess the key Glowforge take-away is that, as with most materials, testing is required with your specific stock to determine optimal settings.