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?

ב"ה Shalom, Your question of eating or drinking before Havdalah is not an issue which arises due to the 9 days but it has to do with any Shabbat. How can anyone continue Se'uda Shlishit after dark? The Shulchan Aruch (או"ח סי' רצט:א) says that one may not eat or drink from sunset on Shabbat until Havdalah is made.!! But if we read on it says in the very same section (סעיף א) , we will see,that if one had started to eat before sunset according to Halacha, one may continue to eat or drink even after dark. So the same applies on Shabbat Chazon , if one has started the Se'uda Shlishit before sunset, one may continue to drink wine even after dark as the Sha'ar Hatzion says .( שער הציון תקנ"א ס"ק עה). Of course, after the person has benched and it is already after sunset , then one may no longer eat or drink until after Havdalah and after Shabbat Chazon may not drink wine. All the best