Sort of, yeah. They're taking a very strict approach to semver - if it could potentially break anyone's code out there, it's a breaking change that warrants a major version bump.
I'd wager that the vast majority of Node APIs currently on 4.x could swap in 5.0 without any issues.
Yeah, why not? Just because they upgrade node, doesn't mean you have to use that version. If you want a stable version, you'll be on the LTS release that will be supported for a long while
The only problem with that is npm is used across all versions. So each version of node that has breaking changes, what do you do as a module author? You might have to drop support for one version or the other or you add in some hacks to try and figure out which APIs you can and cannot use.
Moving fast is one thing and this is the first major version after the io.js and node.js merger so maybe this is a one-off thing; if they continue this pace with breaking changes then the ecosystem is either going to only be latest or are going to stick with 4.2.x forever.
