Just another clickbait article. Sure sign of sinking website.
As for article. Of course, because mining card has cooling issue. Very likely it needs thermal paste replacement. But basically what us happening is thermal throttling, meaning card clicks down to stay withing temperature target. And instead of replacing paste and actually figuring things out to our out educational piece, it seems like author was way too much in hurry to cash in on clickbait. A good site would take article like that down, because they value their regular visitors and audience more than burst you get with clickbait. I guess not this one. Just saying.
As for mining cards, they are safe purchase for reasonable price, especially if you can get picture of mining rig and can see that cards were spaced nicely. Miners will usually even undervolt them so they don't run too hot. But there is no harm in asking about it. Also always be prepared to change thermal paste on used cads. It could be fine, but I would do it just in case anyway.
But even with mining those cards will still last you for years to come. Unless you are looking for card you will keep more than 5 years, it will be fine. And as long as price is right, that is non issue.
Also you can get same with used cards sold by gamers. You never know how well cooled and maintained card was there either.