Folks,
Last time I looked, the US AIP/AIM specified three specific instances of how descent clearances are to be interpreted.
What ATC (apparently) did was in conformity with the AIP/AIM and presumably any manual of ATC instructions.
These three specific forms/interpretations of descent clearances are notified to ICAO as differences to Annex X/Vol.II and PANS/RAC Doc.4444.
As a pilot with non-US operator, I always made certain my fellow crew members or trainees always knew and understood such clearances.
Pilots are expected to conform to any LSA on a procedure. This is a pilot responsibility, not ATC, unless you are on radar vector, then still be very careful about ATC descent instruction in any area where terrain might be an issue.