Just to save anybody else the trouble - I looked up potential instances of 409k in the US Code (in response to Trump tweet). I could only find two 409k sections:
Title 47-TELECOMMUNICATIONS - §409. Hearings - (k) Deposition fees
Title 42-THE PUBLIC HEALTH AND WELFARE - §409. “Wages” defined - (k) “National average wage index” and “deferred compensation amount” defined
Doesn’t look relevant to anything