Why do you claim that so confidently, when many people say otherwise? Are you just going off some metric like DPI?
You're probably missing things like a sibling comment mentioned: "professionally printed books for example use slightly gray letters on creme paper." I don't think you could get "slightly gray" with a laser printer, and print-on-demand seems to basically use bright-white office paper (probably for reasons of laziness and cheapness).
Those people are fetishizing the limitations of offset printing. You simply can't produce sharp blacks comparable to an industrial laser printer with offset printing.
> I don't think you could get "slightly gray" with a laser printer
You absolutely can. But pure black on Natural Shade (off-white or cream) paper looks much better.
Most POD setups use inkjet printers for cost reasons which results in poor print quality.