Your own cite says 4 times as fast. But it is careful not to jump to the implication of a new vaccine required every two years. My suggestion is that you should not jump to the conclusion on your own.
I did not jump, I said this was more likely, which is different. The 2 X comes from twice as large but 4 X lower mutation rate.
Nothing about COVID is certain, we do not yet know any vaccine candidates actually work, but things are a lot more well understood now than they would have been 10 years ago. We have had SARS, MERS, a of lot genome sequencing and study of related viruses. We have had studies of mutation rate of critical conserved sequence vaccine targets in COVID.