Well I went back to confirm the dates and I admit to some embarrassment. The ESOP termination was initiated upon sale of the company, which was 4/2017. Payout of the first 80% occurred 10/2017. The 20% holdback was supposed to be distributed 9/2018. So, only two months beyond the promised date at this point, and a bit less than two years since termination.
Based on what I've learned on this site, despite being delayed, the distribution doesn't yet seem to meet the criteria for excessive delay.
Given that they've provided me with a plausible reason, and based on the timeline in the SPD re: claims process, I'll give it another few months before forcing their hand with a claim. Thanks to everyone for responding!