Precisely ! The F18s would have been manoeuvring at high speed, high thrust, with no engine bleed worth speaking of, and probably at medium/low level. Under these circumstances they would have almost inevitably been damaged by ash ingestion.
Your average high-bypass commercial turbofan, on the other hand, operates at lower speed and relatively constant thrust, with air bleed. Our manuals tell us that in the event of an ash encounter we reduce speed and increase air bleed (and get the hell out of there).
The British government and authorities at the time of the original ash closure also mentioned an RAF Typhoon that had allegedly been damaged by ash. No doubt that was also manoeuvring at high thrust, low level, which bears no resemblance to the civil jet environment.