Your bar for "cryptic" seems a bit low.
The Sanders campaign has been loudly touting the "two million" figure every place I can see. It's on the tip of the tongue of every serious Bernie supporter out there. You posted the comment I responded to all of seven hours after bragging about being one of them. I didn't think it would take a rocket belt to jump that gap.
Particularly since the donation was specifically mentioned in the Shkreli article Mcfly linked, which I foolishly assumed people read. That's how I found out about the donation. That's kind of a damn big breadcrumb.