If that is the case then I doubt the authorities would even need a subpoena just to know their clients' trading transaction. Subpoenas are only served once there is a lawsuit filed in the court whether it is a civil case or a criminal one which in this case the IRS wouldn't need since they are just doing regular background checks if the U.S. citizens that are trading in crypto are paying their taxes right.
The IRS
has no legal right to broadly fish for information like that. That's why they had to fight Coinbase in court for 2 years
just to get data on ~13,000 higher volume traders.
I'm sure Poloniex has responded to one-off requests from the IRS before, but I'm also sure it's nothing on the scale that we've seen from Coinbase. The OP confirmed he sold bitcoins through Coinbase during the tax years in question, which corroborates the theory that it's Coinbase users who are receiving these letters.