If I give a book to someone as a gift I might sign it too. But I would tend to describe it as an inscription not an autograph.
In decades past, particularly when there was still a charge for the literature, sometimes JWs would buy WT books and give them as gifts with inscriptions inside.
"To Betty, love from May, Edinburgh Assembly 1969"
So maybe the GB "signing" Bibles and giving them to Cuban officials a gifts was not all that peculiar, if it was intended as a friendly gesture.
Initially I had the notion of JW "fans" presenting the GB with Bibles to autograph. But clearly that's not the situation.