Sounds plausible, but I suppose that not every disease cured with plants has a microbial origin.

Fig leaves are proving to demonstrate strong anti-cancer properties for roughly half of cancer patients, though more testing is required because those anti-cancer properties could become toxic in excess quantities. That is evidence of plants curing diseases without a microbial origin.

But how can you explain that from an evolutionary perspective then?

Plants are continuously evolving a variety of chemicals to ward off pests. Most stimulants like caffeine and nicotine, for example, are actually potent insecticides.

Among the most interesting of plant responses is plant latex. Latex is found in lettuce, thistles, rubber, fig, and variety of other plants. Latex is thick with a variety of anti-microbial chemicals to ward off fungus, viruses, bacteria, insects, and more. These chemicals provide a wide distribution of medicinal value. Opium is a plant latex.

Have you considered why Australia has the most poisonous animals on the planet? Many venoms found there can easily kill humans many times over. It is supremely excessive, for humans, but not for the other animals living there. It is a constant state of evolving bio-chemical warfare because the intended victims evolve resistances to the strong venoms until they no longer become strong, so the predators and insects continually develop more targeted venoms to keep up.