The author is circling around, but not quite reaching, a statement that POSIX Basic Regular Expressions work everywhere, with the caveat that that not everyone has caught up with version 8 of the Single Unix Specification, which has slightly changed BREs.
I don't think your comment is fair to the author. If there was not such caveat, then there would not be a need to write that article.
The author doesn't actually cover the caveat (3 newly valid backslash escapes in BREs) at all, so that's not the case.