shmendrick – After thinking it over -I’m begining to understand how you or your friend who fed you this got lost. It probably went like this – saying “Good Shabbos” is making Kiddush, once you made Kiddush you accepted Shabbos.
Nope -saying “Good Shabbos” is Not making Kiddush; just R’ A. Eiger had a Sofek if you were Yotzay Kiddush D’oraysa by saying “Good Shabbos”. It works only forwards, not backwards.