benching on wine on Motzei shabbos Chazon by Shalosh Seudos if after night see shaar tzion 551 - 75. Why isn’t there a problem with drinking before havdalah?

See Mishnah Berurah 299, 14. that one who always recites Birkat Hamazon on a glass of wine, can do so and drink it before Havdalah even if it’s after Tzeit Hakochavim. Others, who do it only occasionally, can drink the wine before Havdalah if it is still Bein Hashmashot.