The author is saying linear regulators are better for high current which is questionable. He even references huawei but Huawei chargers don't even use linears. One Plus makes sense because their warp charge is ridiculous, they're doing 5V/6A which isnt even possible using USB PD protocol and theyre using a type a to c cable so clearly doing something proprietary there.
If i had to guess why oneplus can't step down higher voltages, it's probably specifically designed for 5V so it only supports the 5V/3A usb PD profile (hence the ~15W with a PD charger). No clue why they didn't allow 5V/5A for 25W over USB PD since that is possible. I don't think the author mentions anywhere in his article which 60W usb PD charger was used, so its hard to say that it is a valid test. 60W normally indicates 20V/3A, so it's possible the charger could not provide 5V/5A if the oneplus 8 pro did support that.
Anyway, huawei's own PD charger can charge faster over the PD protocol than this guys "60W PD Charger" according to https://www.firstxw.com/view/230511.html.