That point may work against a heavy because how are they going to defend themselves up close with weapons that do splash dmg they will hurt themselves while getting shot by the light.
The thing with Heavies though is that they have access to all the weapons, not just Heavy, splash damage or spread weapons. If the Light comes in close, The heavy could switch to his shotty or AR, or whatever else he has. He could also cook a nade and end it that way. A Light does have an advantage using his speed up close to a Heavy, but the Heavy isn't entirely defenseless. Besides, as mentioned above, if the teams were each being played "perfectly" the Heavies would be in groups, not soloing.