Silicone tongs can handle high heat and as long as there's no open flame that comes into contact with the silicone tongs they can be used on the grill.
I've used my silicone tongs on my grill but made sure to turn the grill down before using the tongs to avoid the open flame touching and melting the silicone tongs.
If you have an open flame touching the silicone tongs then they could melt but as long as you turn the flame down or off while you turn the food over or pick the food up with the silicone tongs then the silicone tongs should be fine.
If you want to touch open flame though with the tongs then use some metal tongs instead.