The Contract of Carriage specifies the on-board time limit.

2) BOARDING TIMES – PASSENGER MUST BE ON BOARD OR PRESENT AT THE
BOARDING GATE-IN ACCORDANCE WITH THE FOLLOWING TIME LIMITS:
a) FLIGHTS BETWEEN ALL POINTS IN THE FIFTY UNITED STATES – AT
LEAST FIFTEEN (15) MINUTES BEFORE THE SCHEDULED OR POSTED
DEPARTURE TIME;
b) FOR ALL OTHER FLIGHTS – AT LEAST THIRTY (30) MINUTES
BEFORE THE SCHEDULED OR POSTED DEPARTURE TIME.
3) WHEN A FLIGHT IS DELAYED AND THE SCHEDULED DEPARTURE TIME

I see the intellectual inconsistency, but that doesn't change contract responsibility or the law that defines when IDB compensation is due.