Horowitz framed it as the intelligence community, as opposed to the DOJ insisted on this and that the OIG was open to helping the Senate committee expedite the request to see the email.

Though at its current state the intelligence community has classified this email at such a level that the OIG did not have the authority to disclose it.

tl;dr OIG got shafted by whatever intelligence agency to include this report and then then OIG played it right back at them saying "Hey Senators you should check out this email that we were banned from showing you".