Once? No jellyfin has had about 4 major RCE issues since the fork. At least 4 that I'm aware of. Blaming it on the previous code only makes sense if the split is recent. They have had time to completely rewrite if they really want.
It absolutely makes sense, otherwise they would have had to throw everything away.
The EFcore refacotring was like 6 years in the making.
And all that from just a few single ppl. Look at the ckntributer list, and how many contribution. Not many active devs are working on jellyfin on their free time. The problems that jellyfin has, is not from a lack of trying but a from a lack of finger and arms.
And you need to take it like it is.