Exactly. At the moment there is no way to determine if a strong player used an engine for a single move or not (without proctoring). And one perfect move in a c

Exactly. At the moment there is no way to determine if a strong player used an engine for a single move or not (without proctoring). And one perfect move in a critical position can make all the difference. Also more subtle cheaters would never boost their performance to carlsen levels, but instead increase their performance slightly (e.g. from 2550 to 2650 a +100 elo boost).