If your code can execute 10x faster without jQuery then the optimization is not premature. If a developer takes twice as long to write any code the problem isn’t optimizations at all.
One could also argue that driving to work is premature if you have two perfectly good legs. All ridiculousness aside a person can invent any absurd excuse to hide from the common practices of their profession.
"optimization - an act, process, or methodology of making something (such as a design, system, or decision) as fully perfect, functional, or effective as possible"
If you optimize process of delivering feature to the end user then using JQuery is optimization and not premature. It does not have to be optimization of runtime/load time.
One could argue that it is premature if the code doesn't _need_ to run 10x faster.