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.
The MMM lifestyle is something like 25-30k to live.
If you make 50k.. you can save at most 20-25k, taking (10-15 years to retire, would need to do math).
if you make 200k, you can save 150-175k, reaching retirement in 2-3 years.
The "higher income -> higher lifestyle" is a myth MMM tries hard to dispel. You don't NEED newer cars because you make more money. You don't NEED a bigger house. Sure you can choose those things...
It has a huge impact if you are willing to approach it that way, but my point is that I don't believe many/most people in that 100k+ earning range actually are willing to retire to a 20/25k range, at all.
Which leads you back to scaling everything up to the number you are willing to you are comfortable with.
