"He assumed that such a top-level player would never make such a mistake"
But if top-level players make such mistakes in about 1% of their moves, that assumption is utterly wrong (1% per move translates to (ballpark) once in every five games that an similarly ranked opponent essentially gives you victory, if you yourself manage not to blunder), so one could call making the assumption a blunder.
But if top-level players make such mistakes in about 1% of their moves, that assumption is utterly wrong (1% per move translates to (ballpark) once in every five games that an similarly ranked opponent essentially gives you victory, if you yourself manage not to blunder), so one could call making the assumption a blunder.