So RAID controller just flips a bit on one drive, not necessarily the correct one? Therefore I don't see any significant advantage of RAID 6 over RAID 5 here.

What happens if there's error on the drive that internal ECC can't correct? Drive returns error message and RAID can use another drive to get the data, right?

BTW do you know anything about chance of getting bit corruption in case of having RAID+ECC mem?
I mean (undetected) error on HDD / in memory / cables etc.?
I guess that if industry doesn't care, it's a very minor thing, but it would still be good to know the problem.