1) there are (since 1 year) restartable builds/stages in open source (a bit over a year IIRC) - when the article mentions that they are not in open source.
2) Jenkins X is something VERY DIFFERENT from Jenkins (despite the name), it is "master less", yes requires a kube cluster to power it, but it has no Jenkins instances as you know it, in a lot of ways quite a bit different as it uses Tekton as the engine for running pipelines (which has a bunch of advantages). So I wouldn't group it in with the same challenges and constraints. It is something new that shares a name and some of the ideas.
Jenkins, along with Spinnaker and Tekton and Jenkins X are now part of https://cd.foundation - worth a look to see how they are evolving (expect some changes).