Event Supervisors should not make up rules. Do you know if that decision went to arbitration? I'd love to know in which line of the rules the Event Supervisor found this interpretation.
The rules say that "participants may designate a timer, an action taking over 10 seconds that does not use electricity or springs for power, to be eligible for bonus points". Nowhere does it say that the action they designate as a timer may not be one of the 12 unique scorable actions from 3c. Each scorable action may only earn points once in the ASL (rule 3e(iii)), but the timer bonus is not "in the ASL".
(From the example ASL on the soinc website (2019 C rules), "1. Do not list bonus points for timing.". Also note that in that case task 7 was identified as both meeting scorable action (i) and as the timer. )
I don't think there is any way in which the ES can have been correct in his ruling here.