I assumed there was higher too. Like you said, the achievement text strongly suggests there is.

I never tried to go higher though. People say it's fast to grind now, but when I did it before nerfs it was a monster grind and the items/feed used to cost a lot, so I only went as far as I needed.

Silver lining: grind isn't so bad now and you can earn MGP fast especially during the current event. But yeah, system could be more clear.