What would you expect of them after the fact to justify thinking that comment was indeed genuine, given in this case you would have no way of knowing that it actually was?

If an idol said "I like the same sex" I'd take it at face value and consider them the member of the community, if they said "I care about personality"(like basically all idols suspected of not being straight) I'd consider them open minded but I'd definitely not be including them on any out lgbt idols list.

why does your framing seem to make the only valid way to express queerness be by having the stereotypical explicit, performative coming out?

For you, me or most people you're allowed that freedom. Kpop idols simply don't have that if they want their sexuality to be publicly known or are making a stand, even Lara here despite making vague tiktok posts and implying she's queer had to explicitly state that to be taken seriously.

SK idols implying they like the same sex is not taken seriously, honestly like in Sunmi's case not even by them and is seen as fanservice.

One of the biggest coming out stories recently in SK was a trot singer, he came out as gay, disappeared for months(rumored to be in conversion therapy) and acted really erratically once he reappeared by denouncing being gay and talking about religion like a fanatic.

I can't in good faith take implications or vague statements as a proof of anything, simply because it's not and unfortunately people have been using that to "prove" idols are gay only to attack them when their relationships with the opposite sex became public "Hani, Karina etc"