I’m having a brain fart at the moment and cannot recall what this thing is called. It’s not the title of the book, but it is the “tagline” normally written in smaller print on the cover. An example something like, “He fought the law - and won!”
Actually, from the example in the OP I’d say no. A subtitle is an alternate title, usually longer, and will normally start with or as in “Dr. Strangelove or: How I Learned to Stop Worrying and Love the Bomb” or “Oliver Twist; or, The Parish Boy’s Progress”.
I think what you have there is a blurb, which is a kind of advertising byline:
Non-fiction works often have a brief main title that is often intended to be eye catching and ‘clever’ in some way, followed by a colon and then a subtitle that actually directly states what the book is about in a duller, more literal way.
I think you are right that what the OP describes is not a subtitle, but I think a blurb is usually something a good deal longer: a brief paragraph at the least, giving either a synopsis of the contents or extolling them (or both). Blurbs are usually on the back cover or the inside flap of a dust jacket.
I think Mr Downtown’s suggestion is more like it, although strap line or catch line are also possibilities.
It’s a matter of interpretation. Some librarians would call them subtitles like whoever has catalogued “Dr Strangelove” for the British National Bibliography. I would hesitate to do it as they look more like some kind of alternate titles.