It isn't true anymore then saying Slack is rewrite of Atom. While it is using electron, editor and IDE aren't from Atom.
> VSCode is basically a rewrite of Atom that reuses a big chunk of the original codebase
They both use Electron as their framework, but that's about where the code similarities end.
Atom and VSCode projects have always been separate projects. Both using Electron does not mean they share a codebase, not in the normal sense of the phrase.
I'm not an expert on this at all, I'm just quoting, but Spark is being used rhetorically as if it's an outlandish to consider it to be a descendent
One should make a difference between (1) access to the source code to look, find bugs, make minor adjustments and (2) everybody with that access to be able to actively participate in the development.
In the sense of (1), open source is "some part of a commons". That's what the license gives you.
In the sense of (2), open source may or not be open to a community approach. It's fully legitimate for some company or other group of maintainers to provide (1) but deny anything related to (2). You are free to fork and make your own community, but there is nothing that guarantees you that your pull requests are accepted or even looked at, bug reports reacted to or anybody listening to wishes, concerns or other opinions about the project. It's nice if that's provided, and one could argue that some part of the spirit of open source is to enable a community, which in turn helps the original creators (and could be a major motivation to open source something), but nobody should be upset if maintainers choose otherwise.
It's nice if it's clearly communicated how maintainers see this though.
Instead of letting it slowly bleed out and randomly catching stragglers with outdated marketing materials lingering on the web, they give it a swift death. Atom can go into the history books as an open source project that defined the industry for a decade.