This doesn't make a lot of sense to me, and it doesn't seem to be backed by any evidence, either in the original Twitter thread nor the linked blog post.

I think far more likely what's happening is Pinterest is actively crawling the web for images and surrounding text. They will then perform their own reverse image search on _their own_ database, and add this context text to their existing copy of the image, or create a new one if one is not present.

Why would they rely on a user submitting a picture then go to the trouble of reverse searching it? I think they are much more proactive than that.