Some of the words chosen are rather absurd/inappropriate: breviary (which I got wrong but felt like a vaguely religious word) was characterized as intermediate but I think it's much more obscure and less obvious than that; Hippopotomonstrosesquippedaliophobia was used as a word (I got that wrong as well) - any type of 'phobia' word is really the sort of thing a fourth grader opens up a page in the dictionary and points out, not a word that is used... ever; metamorphosis and kinetic were labeled expert, which I don't agree with (what elementary schooler doesn't learn about the metamorphosis of a caterpillar into a butterfly? what high schooler doesn't learn about kinetic energy?).
Most words were reasonably well defined in a way that most people would understand or recognize. A few words had poor definitions: lethargy ("the state of being lethargic" - obvious); complacent ("smug satisfaction with oneself" - I disagree that complacency is intrinsically smug); magnanimous ("generous toward a rival" - I disagree that a rival must be involved); gauche ("socially awkward" - this is sort of close but the given definition completely misses the idea of being tactless).
They call it scientific and give a hand-wavey formula, but they don't explain how words are stratified in the first place. If stratified sampling is a formally recognized method of doing this, it would be nice to have a link to a real reference. I think I know a lot of words, but I am skeptical of the estimate this app provided (north of 75k).