What's the LX ex theorie ?

Like I was swiping on tiktok when suddenly a video with the music of F R I E N D S (marshmallow) with LG and LX in it, suddenly LG was snubbing lX (like it's his ex) It had the caption "lowkay those LX ex theories) When I went to check the comments, I know that this theory came from the Chinese community, but no one explained it ,so I would like to know what this theory is about ? (Maybe because people saw that the first time they were seen interacting, it echoed that they already knew each other)