From what I can gather there is nothing that is going to harm your Cham from feeding earthworms occasionally.
it seems the biggest risk is with parasites but I don’t think earthworms would have many parasites that could effect a Cham. The source of the earthworms is a big factor in parasites. Seeing as yours are timberline I would guess there is a lot less chance of them having parasites.
they are on the not recommended section but so are butterworms and mealworms and I feed both of those on occasion. It’s all about variety. With butterworms there is the risk of allergic reaction in some chams though.
that being said I have never fed earthworms to my chams because there are better options.