I disagree. Michael Phelps is double jointed. He's the best swimmer in the world because he has a mutation that makes his feet more effective flippers. You said a flaw is still a disability even when everyone has it. Nearly everyone is single jointed, and that makes us worse at swimming than Phelps. Your argument would imply that single jointed people are all disabled.
You can't define disability in absolute terms, or you'll run into problems like that. You have to define disability in socially constructed terms.