Looking back, it appears there is some truth to that. Amarin received a CRL on April 27, 2015 (hyperlink). On that day, the FDA denied all 22 citizen petitions (see FDA-2013-P-1612-0429 thru FDA-2013-P-1612-0453).
Currently, there are two citizen petitions still open that were submitted by MRC. While keeping an eye out for denial letters may warn us in advance of sNDA approval, I don't know how predictive it will be if history repeats itself and they accomplish both on the same day.