When people talk about a language being open source they primarily mean the compiler. A closed compiler surrounded by a collection of open source things is an anti-pattern and trap to be avoided if you're interested in open tools, that's why open sourcing the Mojo compiler was so important to many people.
> Chris (also created swift) talked in the past the reason for not fully open-sourcing was more because he wanted to get all the core design decisions right.
Chris literally was saying it from the very beginning (you can watch early interviews, obviously he was being asked about it), everything that happens with progressive opening is what he said he'll do and he gave rationale for it which is summarized by what parent comment says.
That doesn't seem to address what the parent comment says; they're arguing that the rationale for the progressive opening doesn't make sense, not that it didn't or wouldn't happen. If I say "I'm going to work on feature XYZ at work today because the sky is yellow and grass is purple", it would not make sense to defend my claim tomorrow by pointing out that I did work on feature XYZ.
> Chris (also created swift) talked in the past the reason for not fully open-sourcing was more because he wanted to get all the core design decisions right.
This I don't buy so much, as there seem to be a lot of core design decisions still up in the air: https://mojolang.org/docs/roadmap/