From the source's mouth: "I've never worked with such a great team and learned so much in such a short period of time. I owe all of it to the opportunity you've given me, Jason and I thank you immensely for that."
So we have a one-sided story, from a source who contradicts himself. I'd rather not jump to conclusions just yet.
Where is the contradiction there? It seems like a standard "it's been great but I have an even better opportunity to go for" email.
In fact; it could well be complete bunk. He might have hated working there. But if so it's an example of how to deal with these situations - polite and positive.
So we have a one-sided story, from a source who contradicts himself. I'd rather not jump to conclusions just yet.