Regarding the second half of your question, we don’t guarantee any particular range of motion, and in fact there’s some variation on maximum physical travel from head to head. We mistakenly put a specific range of motion on the website before, then realized the error and corrected it.
Regarding the first half, the focal point of a lens is infinitesimally small; any optical system “in focus” will necessarily have some distance between the focal point and the surface. I don’t know of any universal quantitative definition of “in focus” across all optical systems; it typically means “close enough to the focal point that the desired goal can be achieved” - for example, an in-focus picture is one where bluriness isn’t noticeable, or is below a certain wavelength, or grain size, or somesuch.
Interlude: just checked Wikipedia to make sure I wasn’t too far off base. It says:
An image, or image point or region, is in focus if light from object points is converged almost as much as possible in the image, and out of focus if light is not well converged. The border between these is sometimes defined using a circle of confusion criterion.
It seems to agree that the definition is subjective, although it doesn’t propose a functional test.
In any case, the Glowforge lens can move sufficiently to place that focal point close enough to the surface of material to cut (up to 0.25") or engrave (up to 0.5"). Since it can focus sufficiently to accomplish its goal - yes.
For those curious about history, we first designed it so it can engrave 0.5" materials; then later, wrote the website copy as best we could to convey that, as well as address questions like “well, can I engrave over the full 2” height of a 2" tall object?". As always, our goal is to try and convey the best information possible to the largest number of people, and we may occasionally fall short. We did initially (with range of motion) but I think the current language is accurate and leads people to the correct conclusion.
I think this was a practical question about what you can do with your Glowforge, which I’m happy to do my best to answer. In the unlikely event that folks want to use this as an opportunity to debate the definition of technical terms, I will respectfully allow them to do so without interrupting. 
