Is mindfulness a type of meditation?

Not quite, and the field itself is split. The tradition most courses and apps descend from treats sitting practice as the main route to the quality the word names, which is why the two are usually taught together. A separate tradition, developed by Ellen Langer from the 1970s, uses the same word for noticing novelty while doing something else, with no sitting at all. Their scales correlate in one small preprint study, but the two were built from different foundations.