I didn't really mean "chosen one" because of his forehead scar or anything. The flashback showed someone who looked like his father (but wasn't, I assume) who opposed Muzan in the past with his sunbreathing techniques. Tanjiro is related to that guy, so now he's the chosen one in so far that he's the descendant of someone who also fought Muzan. That's shit just like it was shit in Naruto when we found out that our hardworker/outcast was actually related to a whole bunch of historically important heroes.
Again, I don't care too much about that anymore, I'm used to it. But let's recognize it.
As for sunbreathing: If it really was sunbreathing, shouldn't he have been able to cut off her head even despite her loosening up the material? Sure, the physical sword edge wouldn't be able to cut, but the sunbreathing effect should still dissolve her tissue like a chemical reaction?