Dick, The Benoni who died in 1819 was the grandson of Benoni, and brother to Seth Jr. who you identified in your research.
But be careful...The Twitchell genealogy confuses this Benoni (T208) with a 3rd Benoni (T439).The Dudly vitals list Benoni (T208) as the son of Seth (from Thompson). The Athol vitals list Benoni (T439) as the son of Enos and Relief. There were two significant marriages recorded in Athol, namely, Benona Twichell m. 1792 Leah Stockwell, and later Benoni m. 1800, Zilpah Woodward. The first marriage almost certainly was not Benoni (T439) because he would have only been 11 yrs old. The last known record of Benoni (T439) is the 1820 New Salem, MA census.