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.
I feel like the simplest way to fix this issue is to just directly label any visual and play issues. If it’s “new and in box” then say that. If it has no visual issues and no vibe, say that. If it has scuffs or dings or vibe, take pictures of any and all blemishes and state the issues with how it plays. Using a single sentence is so much more useful than just “mint” or “near mint”. Those shorthand ways of talking about condition are kind of useless when some people decide that those shouldn’t apply to their listings.