Even though I also enjoy not closing my li's, I don't think this is an egregious error. The app appears to be focused on helping novices interested in learning HTML. To those just beginning, it's probably most important to enforce the idea of most elements having opening and closing tags.
However, if their target audience is actual developers...well, they've got lots of other problems to deal with as well.
However, if their target audience is actual developers...well, they've got lots of other problems to deal with as well.