You're giving Giridharadas way too much credit. His e-mail resignation is an implicit ultimatum (if you don't fire him, I will go public), to which Hoffman counters by saying how he will try to frame Giridharadas as trying to personally gain from this scandal (which, clearly he will).
By publicly stating Hoffman's weak threat, while reserving the future threat of revealing their correspondence, Giridharadas makes it harder for Hoffman to follow through on the threat without public backlash.
So Giridharadas targets this tweetstorm to inflict maximum damage to Ito supporters and MIT admins. The hush money line is harder to prove and channels frustration towards the hushers more than the facilitators. The cash for prestige line places all the blame on Epstein and Ito. Then he publicly shares the list of Ito supporters [0] and creates public knowledge of their thinning numbers. When the firing eventually occurs, Giridharadas will have collected the spoils of a successful cancellation, the public knowledge that he can make very credible threats.
If you are correct, we should expect Giridharadas to place more emphasis on the hushers for the sake of justice. If I am, we should expect him to put all effort into bringing down Ito.
[0] https://twitter.com/AnandWrites/status/1170025167063605250