I don't think illustrations necessarily have to accompany or be a part of a larger project. Illustrations can stand alone. I'd say the only real criteria is some narrative quality, but even that can be blurred a bit. There is definitely a large gray area between Illustration & fine art where things seem to be both to some degree. Some pieces are no question one or the other, and some might fit both.
Also if a portrait of someone's face can be an illustration, then why not a "portrait" of trees or a forest. Still lifes can be illustration. Landscapes can be illustration.
Before the rise of photography, it was common for illustrators to paint photorealistic products for advertisements, etc and those were definitely illustrations. People still do that today.
I'd say that the primary thing is a narrative aspect whether it is blatant like, "The little girl in the red Hood picks flowers as the wolf stalks behind" or just trying to capture a feeling. Illustration is a commercial art focused on a technical artistic skills used to solve a problem. It's on the spectrum between design and fine art for sure.
Also, lots of the paintings of the Old Masters contain the same aspects of Illustration of today that we admire, but they aren't considered illustrations.
TL;DR You decide. I'd say yes it is.