Another thing to note is that evolution works on everything all at once. The fact that a chemical is poisonous to mammals can be coincidentally when the real purpose is for something else. Fungus have to do lots of decomposing and digesting of different compounds.

Also to note that the mushroom part is just the fruiting body for the fungus that spreads the seeds. So poison there is under a different evolutionary pressure than a poison frog who will die when eaten . If the fruiting body spreads spores then it’s done it’s job