It's because he helped so much it's suspicious. Truthfully there aren't many candidates as to whom is being impersonated by Liar. Fuego Vermillion is not possible as he's in a coma and was not present during the meeting. Yami, Nozel, Jack and Charlotte are also out of the picture as they all fought against Liar in the cave. Unless of course the switch happened afterwards, but that's unlikely considering as then Liar couldn't really have done much sabotaging or leak much information and ergo there wouldn't be a reason to suspect another traitor. The personalities of all these captains are also quite outspoken and till further notice Liar might be able to copy magic and looks, but not personalities and memories. So it should have been obvious for other people if one of them was acting out of character. Poizot is not possible either as he had already been umasked as a traitor and taken into custody.
So that leaves us with only tree options remaining: William Vengeance, Dorothy Ansoit and Lil Bowamoltie. Now Vengeance has been acting suspiciously, however this is something Yami himself addressed in the series. He's simply too obvious and feels like a Red Herring (though I still think that he's inadvertently connected somehow to the White Demon Eyes). So most likely it's either Dorothy or Lil and the reason why I prefer the latter, is because he was so helpful. Liar admitted himself he is quite lazy, but he still was proud of his acting performance he demonstrated during the meeting. Rationally speaking Dorothy is probably the easiest one of all to impersonate as the only thing she does is snore. She didn't do anything either to help when Poizot tried to escape. Liar however claimed that they wouldn't be suspicious for a while of him, which implies he did something to gain their trust.
Now they probably didn't had any use anymore for Poizot, even more so considering they already had a 2nd person infiltrating the magic knights and thus he was disposable. So aiding the other captains in capturing him could be an attempt to divert suspicion from Lil. The fact that Lil also actively approached Aster is suspicious as they made it obvious they want his grimoire.
