Oh yes, Eusebius was a great and reliable historian.
So, Eusebius records this legend as a part of the history of the church and that disqualifies him as an historian?
Was the legend in existence or not? That is the question. The letter existed, there were those who believed it, including Eusebius. This has happened from the beginning. The Church does not promote individual revelations as binding on all her children.
But, this goes to the same process by which Canon was determined.
There were hundreds of writings and several more gospels than were included in the Canon.
The church sifted through them and then the Holy Spirit led them to separate the wheat from the chaff.
Those that were found to be Scriptural were included, most were not.
It doesn’t change the lineage, or disqualify the succession he records. Protestants don’t understand that not everything written is infallible. Or maybe they do, but don’t care to know the truth.
JV — remember, Unitarians are not Protestants. They’re not even Christian.