Please note the use of "could" and the following:
"Some cryptominers have told us that they will undervolt graphics cards and optimize them to be efficient rather than simply max them out. As such, a graphics card used for high-end gaming hours on end could be in a worse state. But we'd not advise you buy one of those either.
"And while individual miners may optimize their GPU for efficient mining, there's no guarantee big crypto farms will have done the same. In short, proceed with caution here. "
Also, these cards MAY not have been tuned as the mining farms look to make a quick crypto buck so may be more inclined to deadline cards to get a faster ROI. We're talking about bulk users, not individuals who will have more likely tuned the GPU.
Either way, buying these cards at full or more than retail price when they aren't new isn't ideal for the average consumer.