Nothing made of atoms is truly chemically inert, not even noble gases. It's just more or less reactive, and when/how.
But even if it was literally un-reactive, sometimes it's enough to just be in the way. Imagine folding a protein, or assembling a structure of RNA origami*, but some big lump of un-reactive molecule is in the middle — the ultimate shape is different, leading to different biochemical results. Grit in the gears.
Or even just heavy: deuterium is chemically identical to hydrogen, but still has a lethal concentration** because it is twice the mass.
** Replacing 50% of the hydrogen in a multicellular organism with deuterium is generally lethal, unless this is a widely believed myth that's about to get a bunch of debunking
