...or just use JAX-RS, which is simple, standardised, and has good implementations (such as RESTEasy and Jersey).

I suppose but SparkJava has as its core philosophy, build as a runnable jar from the start which is in line with what the article was discussing in its referential prohibition of conventional app server approaches.

edit: Further to the point of getting Java server technology to a simpler to understand, design, debug and maintain place, SparkJava makes it insanely easy to do REST services eliminating both Annotations and heavy configuration. I've also speed tested my side projects on SparkJava and they are unreal fast.