The "fraction" refers to expense rate versus savings rate. If your "absolute amount" is low enough relative to your income, your savings rate is high so you can retire earlier.

But that's the point. This process is "faster" on a higher salary if and only if your final target income isn't also higher. For many people this will not be the case, e.g. the person with a 100k/yr salary has a higher retirement cost of living targeted than the person with 50k/yr salary does.

Of course, if you are happy living a 50k/yr lifestyle and retirement but happen to make 100k/yr, yes, you should be able to retire much faster.