1. Saying we use 10% of our brains doesn't imply 90% of the mass is unnecessary. It could be saying 90% of the electrical activity is unnecessary, or 90% of the synapses are unnecessary. I don't believe the myth, but doing a hard logic stop from 10% to that means 90% of weight is unnecessary is not implied by the statement we are arguing against. It's nothing to do with arithmetic.
2. If you look at the article one of the 1st headings is "Size Does Not Matter". So does it matter or not? When you look closer the stand out features of humans is a wrinkly grey matter, so surface area (i.e. grey matter) seems important and mass based arguments are a correlation to that. (also big bodies have more sensory area so their is an inverse correlation to body size too)
Anyway, I think the point of the myth is to say humans have more potential than their default. If you take some Ritalin or Cola you might agree that the natural brain can be souped up a bit cognitively (not by 10x). Experience and learning also gets more out of an (nearly) identically sized brain too. You can't take an experience brain and cut half of it out to become a toddler again though