It's not false, just poorly worded and misinterpreted. A legitimized reason for an at will termination would be anything outside of being fired for belonging to a protected class. I don't feel it's legitimate for several reasons. First being that they never declare that consumption outside of work + on call hours is off limits; Second, that this was the exact reason he cited for terminating me; Third, there's a clause in NY Labor Law (which I may be misinterpreting, hence me bringing it up here) that appears to protect against this:

b. an individual's legal use of consumable products prior to the beginning or after the conclusion of the employee's work hours, and off of the employer's premises and without use of the employer's equipment or other property;

https://www.nysenate.gov/legislation/laws/LAB/201-D

I've added it to the OP, which I should have done originally. Sorry for that.