I don't see a contradiction. I read it that the microservices are independent and thus can build in parallel, if several teams work on changes to several microservices.

Spinning a build worker outright when a change us pushed is the fastest way, and may be expensive if the build process is prolonged.

OTOH I've seen much faster image build times, with smart reuse of layers, so that you don't have to re-run that huge npm install if your packages.lock did not change.