I just took another look on Max/Lando Max/Oscar overlays from Q3 in Japan. Track temperature was much lower just at 29oC, but the overall picture looks very similar to Spain. Max is either on par or better in low speed(than the faster Mclaren driver in particular corner). Biggest Mclaren advantage comes from 150-200kph corners just like in Spain. At the same time both Mclaren are faster down the straights when not using the DRS and had higher top speed (without DRS) on the back straight. All points at the superiority of their aerodynamic package. Yes, they might have a trick or two up their sleeves with regards to rear brake temperature, but that's not the reason behind their superiority on classic aero dependent race tracks.
Their main underlying advantage behind their pace, especially behind their race pace is base aerodynamic superiority. They simply have more downforce in the crucial speed range for the same drag.
This time, probably for the first time all year, at least one of their drivers had a truly decent Q3 session, having both clean mistake free runs, improving significantly on improving track and not make any mistakes whatsoever on a pole lap. That's why we got that gap, second largest only behind Australia. If that was the case in Japan, they would be same 3 tenths up easily, at least 3 tenths up.