I too have been wondering if this is mostly placebo. For familiar words, don’t our brains automatically read entire words as a single fixation point? I.e. we don’t read in terms of letters, but see the word as an atomic unit?

I would guess that my brain, by seeing certain words hundreds of thousands to millions of times, has found a more efficient way to identify it than the same word with a few letters bolded. Of course, this is based on no data, I’m genuinely curious if this helps.