As others have said, you can’t compare condition/grades on a 1-to-1 basis between different hobbies because there’s too many variables. A coin is scrutinized to a different degree than a trading card, for example. You can have a PSA 10 Blastoise despite that card being found in a standard booster pack, thus ‘circulated’. As I understand it, you can’t have a ‘mint condition’ (more or less the equivalent to a PSA 10 card) coin that has been circulated, so right there the comparison falls apart.
I’ve never even considered the QC aspect of yoyos. You’re likely to never get a new, UNUSED, high-end yoyo because they all go through QC. One could argue that even screwing the axle in causing microscopic amounts of damage to the threads, thus no longer 100% mint, new condition. Likewise, a ‘mint condition’ classic automobile likely has been driven, despite any sort of mileage causing a small amount of wear to many components.
I wrote this guide many years ago, and people seem to agree with it mostly.