Both of those defenders were linked to a transfer to Bayern Munich a few weeks or even months ago as Bayern needs a replacement for Süle because he is leaving Bayern to join Borussia Dortmund next season. I think Bayern needs another central back now because last year they already lost Boateng. It is already clear now that Rüdiger will not go to Bayern as he already joined Real Madrid and i also did not hear any rumors or news anymore that Christensen might join Bayern. It seems like Bayern will not sign an additional central back after all, which seems pretty weird to me.
Yes Bayern has a great central defender with Hernandez but one is not enough obviously. Bayern will probably play with Pavard or Nianzou as the second central back.