That's a retcon. There was no indication that he was a hobbit in The Hobbit (and as others have mentioned, in the original there was no physical description at all.)
This version says it’s the 1937 edition. It has the pre change story about Gollum offering the ring which Tolkien said is what he changed. But it also says he was a small slimy creature.
But apparently there were dozens of different versions that actually ended up in print that had different amounts of the changes caused by some printers mixing old plates and new. So it's entirely possible that small slimy appeared in some versions around 1951 but not others and that's what that page is working off of.