The paper in question doesn’t do a survival analysis like it ought to for truncation, but it does note that
Table 1 shows that the average age of peak creativity is around 39 years. At the same time, for the 97 of the 99 composers who have died, the fraction of their life span when they created their best work is in the range 0.60 to 0.62.
It would be difficult for mortality bias to meaningfully affect the numbers given that most of them do seem to far outlive their best work.
The paper in question doesn’t do a survival analysis like it ought to for truncation, but it does note that
It would be difficult for mortality bias to meaningfully affect the numbers given that most of them do seem to far outlive their best work.