That's a nonsensical line of reasoning. Hard interviews don't reduce salaries, they increase them. Why do you think FAANG companies pay developers 300k+/year when there are so many lower-status companies who pay <100k/year and still manage to find hires? Because the latter are willing to hire candidates who have worse resumes and less impressive interview skills that the FAANG companies consider beneath them.
You might want to re-read your own post because whether 300k/year is a lot of money or not is a non sequitor. You claimed that companies ask hard interview questions to depress salaries, which is a trivially falsifiable claim when you note that companies that don't ask hard interview questions can get away with paying less money than companies that do because they are less picky about who they hire.
