Well, one of the possibilities is that Tether was printed at a higher rate since October 2017 to keep up with the demand for a cash equivalent asset to help the crypto portfolio managers manage risk on their funds and to reduce the overall volatility of their portfolios in order to generate additional alpha. (Modern Portfolio Theory)

A possible reason for Tether not disclosing the USD account(s) with over $2 Billion is that they may be holding the funds in crypto assets in order to generate even more profit, which is kind of risky.

They may also not want to specifically disclose their bank account(s) info as to avoid potential action by regulators as well as cyber criminals.

Just opining on other possibilities of this Tether scenario...