I think that the confusion may come from the fact that Sony refer to something they “linear IRE” in their white papers, where 20% linear IRE is equivalent to 18% reflectance.
This is further confused by some of the Sony S-Log papers giving conversion equations to reflectance, and some to linear IRE. So sometimes a compensation factor of 0.9 is needed, and sometimes it isn’t.
As far as I know, only Sony do this.
It looks like your plot comes from LUTCalc by Ben Turley. Since Ben (I believe) owns Sony cameras, and built the app originally for his own use, he incorporates the Sony linear IRE calculation in the app.