From the situation your giving, they engaged -BEFORE- giving an RP Notice.
And thus, broke a rule.
Unfortunately the only way to prove that series of events occured. Would be to video it happening. or be able to get a screen of them taking out your shields before their Engagement Notice shows up.
Engagement as I read from the rules is based off of shields being damaged below 50%, ship rammed, or a CD launched.
Offtopic Edit: Jansen.... My my... a gunboat? No more fighter for me to SN from 4k as a "HELLO!!!" anymore?