I always find this sentence to be a bit of a laugh. It's so commonly said (by either group of people with a dog in this fight) but seemingly so uncommonly thought of from the other group's perspective.
People that prefer microservices say it's easier to change/rewrite code in a microservice because you have a clearly defined contract for how that service needs to operate and a much smaller codebase for the given service. The monolith crowd claims it's easier to change/rewrite code in a monolith because it's all one big pile of yarn and if you want to change out one strand of it, you just need to know each juncture where that strand weaves into other strands.
Who is right? I sure don't know. Probably monoliths for tenured employees that have studied the codebase under a microscope for the past few years already, and microservices for everyone else.