In xianxia, possibly also wuxia, cultivators can force a poison out of their system, once they are familiar enough with the poison. Assuming the poison doesn't kill them first or isn't worlds above their level of cultivation. I guess these might be true for this Korean murim world, if it gets magical enough like here. In any case, that poison expert apparently only used variations of the same sort of poison, so if you know one, you basically know them all. Woo-Moon could force them out of his body.
no it's fucking moronic, poison users/clans are feared in every single xianxia known to man, unless you're one of the handful of people who get the "10 thousand poison immunity" bullshit, and even then, it doesn't save you from the titular "poison god" of the setting. Guess why it is so ? Because it's extremely hard to defend against them, it's usually demand expensive and/or ingredients/pills/doctor, or you have to stop and meditate to expel it while being a sitting duck waiting to be killed. This guy is a unparrelled master, on the same level of skill as the MC. Do you think he would bother using poison if any chump could counter them like that ? The writing is just terrible and the mc gains power-ups as the plot demands without rhyme nor reason.
You didn't need to even be a martial artist to know the guy was lying, he was such a lousy actor, so sensing fluctuations in his qi would be merely the cherry on top of the cake. If there are physical signs when a regular person is lying (not visible when a psychopath is lying), then it makes sense there would be even more so in the qi.
Please, we're not talking about acting skills or whatever (which are stated by the mc to be excellent by the way), we're talking specifically about "oh mc could sense he's lying because he can read his true nature through his ki". He's never been able to do it before, nor any other martial artist for that matter, he just gained it right now when it's convenient for the author to point at him and say "look, he can do this now, look at how badass my mc is". It's just beyond stupid.