The goal of google image search is to let you find images you want, not to rank websites by popularity. Pinterest makes it impossible to click through to the image.
It's a bit more than that. (Or all newspapers behind a paywall would be blocked).
The problem is that, even with a logged in Pinterest account, clicking on a Google image search result doesn't take you to the image. Sometimes the image will be on the page, but not always. Sometimes have to perform another search on Pinterest to find the item.
Here's an example where the image is below the fold. There are plenty where the image can't be found without a search on Pinterest.
Why? The image is visible in the search results. You search for images, you see images. What requires a signup is the Pinterest-specific content associated with the image.
When I do a Google Image search, and click the thumbnail which shows me the Google image card, and I click the "visit" link I expect to be taken to a page with the image on it.
... because you didn't read the description of the problem on the Reddit post.