As I've already said Scott, I don't think it is a 'stretch'.
I mean, if James Swanson believed that the book was sent directly by Anderson to Donald Swanson under cover of a letter dated December 1905 then, knowing that there was an inscription in the book, why wouldn't he have assumed that the inscription was written by Anderson?
What surely would be a 'stretch' is any other explanation for this. I noted that Simon didn't provide one, nor you. Fancy it?
Let's leave aside that for all James Swanson knew 'Fred' could have been Anderson's middle name or nickname, because while it obviously says "from Fred" the question is: did James Swanson remember that?
Yes, sure, he once saw it said "from Fred" but then many years later I suggest he forgot this detail and, being aware of the letter from Anderson, simply assumed that the inscription was by Anderson.
If you think memory cannot play tricks on us then you will no doubt think it is a 'stretch'. But I believe it's fairly well established that memory DOES play tricks.
No alternative explanation offered I see, to my satisfaction.