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.
Most POD presses actually use inkjet because it's less expensive. The result is much lower quality.
Can you link to some high-resolution comparison scans to support this claim? I find it hard to believe that any toner-based process is going to result in a cleaner page and crisper text than a properly made-ready offset press run.