I did a lot of research on this prior to HT.
My understanding is that the main reason smoking tobacco is contraindicated for HT or basically any type of surgery is because nicotine is a vasoconstrictor. This means it constricts the veins, raises blood pressure, and can cause less blood supply. It’s why smoking is associated with heart disease and high blood pressure.
Cannabis/THC is generally understood to be a vasodilator. This means it opens the blood vessels, increases blood supply, and lowers blood pressure. It’s why THC can be a wonderful medication for glaucoma, for instance, as it can reduce intraocular pressure. In theory, this may be good for newly placed grafts.
My understanding is that the act of lighting any plant on fire isn’t really the thing here, it’s the blood flow effects of the active ingredients.