I had exactly the same issue - 10 minutes into baking, this melted into a puddle of cornmeal. I think the recipe contains a typo. Made a second batch and baked at 200 degrees for the time specified, and got crisp on the outside, soft on the inside lovely polenta bites. Not sure I'd quite call them croutons, but still delicious in a salad or just to eat out of hand.
Wow. I've made a lot of croutons over the years, but this was a new experience. I've never had anything run together quite like this. 10 minutes into the baking, and it was a single mass of cornmeal. I let the polenta cool for a couple of hours before slicing, but even then it seemed very soft. That is probably putting it mildly.