Isidore of Pelusium did not pirate Eusebius

Somewhere I read that book 2, letter 212 of Isidore of Pelusium was an unacknowledged copy of part of Eusebius Ad Marinum.  This would make it valuable as a witness to the text of the latter.  But I sent the text to the translator today. He has just informed me that in fact it takes rather a different approach to the same bible difficulty — how is Jesus dead for 3 days — and is not part of the Eusebius text.  We’ll translate it anyway, but I need to go back and find out who said that it was.