His claims were not hastily disbelieved. That is not how history happened there, they were both taken as total truth and disbelieved. His claims were disbelieve for various reasons, notably that he extracted quite a few admissions under severe torture. Notably, disbelieved also by his contemporaries who watched it and wrote horrified reports. He tortured also because gold seeking, not just for human sacrifice cause. They became paranoid and over drunk with unlimited power. When people in group X kill each other and then you kill them all, you was unlikely to be merely horrified and trying to help victims. Your motivations are guaranteed to he more complicated, as much as believe in your good intentions make us feel good.
You can have human sacrifice without it being impossibly large and without it being treated like adventure movie plot and without jumping on every fantasy anyone ever wrote - which is what up thread is. You can belive scientific consensus which is based on multiple sources and findings without making feel good stuff about conquest.
And while you are expressing outrage over exceptional kills, it is fine to ask who the source is. Because it does put things into context as it turns out the report is not by independent third party, but by highly motivated player in political and violent struggle for control.