Ummid has more on the back story, with this noteworthy detail:

However, some supporters of the Zionist regime has doubted the veracity of the letter and called it “fabricated” and “Hamas propaganda”.

The Israeli government has barred the reporters from asking questions about the conditions in which the hostages were held or their physical and emotional well-being.

Really is suspicious that the Israeli government isn’t letting reporters talk to the freed hostages and thus not allowing any corroboration (especially after those hostages released earlier said they were treated well).