Well, the About page says that it’s intended for elementary- and middle-school students, and foreigners who live in Japan. It adds that all kanji have hiragana included, and difficult words have dictionary explanations included.

So, not advanced-level, but not quite basic either. Until you’ve got at least a few TextFugu seasons under your belt, you’re pretty much at a pre-school level of Japanese. I reckon it’s a good goal to aim for, though.