Probably because BitKeeper definitely was proprietary back when the Linux kernel was using it even if it got open sourced later... Your comment seemed to be suggesting otherwise.
I specially looked in the dictionary what the translation of "mittlerweile" [de] means in English ("by that time"). Additionally I did not write "BitKeeper was by that time open source under Apache 2.0 License", but "BitKeeper is by that time open source under Apache 2.0 License", which to my understanding of English grammar means that I am writing about something that currently holds. Additionally the German word "mittlerweile" (which by my dictionary means "by that time", as I already remarked) has exactly the meaning that it was different in the past - and my language feeling says this also holds if "by that time" is used in a present tense sentence. Even if not: if I were to say that it was already open source in the past (when Linus Torvald decided to create Git), I would have used the simple past tense. Using a present tense sentence for such a formulation does not make sense with respect to my understanding of the English grammar.
Regardless, you haven't explained what your actual point was. BitKeeper switching to open source after Linus's initial decision to use it doesn't refute the original commenter's point.