But here we are reading it on a general tech news site.

Thats not the fault of the R developers who wrote a message appropriate for the audience to which they sent it.

And they never imagined that the announcement of a major release of a popular piece of software would go beyond the mailing list?

Should every message be written with all possible audiences in mind?

It's lucky I never said that because obviously that would be stupid. However the release of a major version of a popular piece of software should be expected to go beyond the mailing list - and guess what, it being on the front page of HN all morning proves that exact point.