The problem with the Honda terrible narrative of the Mclaren years is that Red Bull was managing to win races with a similarly unreliable and underpowered Renault engine. Mclaren was never bagging podiums, poles, or wins at low speed or high altitude like RB was. Their chassis was as much of an issue if not bigger than the engine.
Perhaps they just assumed the PU deficit was larger than it actually was (due to pompousness), and it led them to believe the PU was the main denominator when it turns out that the main problem with the Honda was reliability, and to a lesser extent, the total power level. If you assume that the other PUs are more powerful than they actually are, and ignore the chassis efficiency of those teams, then you be led to the false assumption that the PU is the issue, and not the chassis.
Look at the straightline performance of the Red Bull vs the Mercedes this season. PUs within 10-15hp of one another but you could easily be fooled into believing the Mercedes PU was severely underpowered.