these are basically spoilers if you haven’t read the fic so i hope you’re okay with that! but basically, what breaks felix’s fever isn’t seungmin himself, but their dynamic/bond together. in my head, what felix desires is physical intimacy with someone who he loves that loves him in return. he does NOT need a romantic element for his fever to be broken, he just needs to be physically intimate with someone who deeply loves him. that’s why seungmin is able to break his fever (but also why seungmin’s fever cannot be broken by felix, because seungmin needs a very specific type of emotional intimacy from one person). anyone in skz could have broken felix’s fever, seungmin just happened to be the first one there to do so. but of course, by the time chan arrives felix and seungmin are hiding this knowledge, because felix decides to use the pollen as a way to get chan to be honest with him (and himself.) no one ever tells chan, they keep the fact that seungmin and felix were intimate as a secret forever imo!

thank you for asking and i hope this answers your question:D