The tank will get hot from engine heat regardless of whether fuel is sloshing around in it. Ask any Brick owner. The previous owner might be referring to vapor lock which is a real condition. When overheated gasoline turns from liquid to vapor in fuel lines, the increased volume of vapor can interfere with fuel flow and performance.
Another condition: When heated, fuel and fuel vapor expand regardless of whether they are heated inside the tank by the sun shining on it or by the engine when its running. When fuel expands too much without enough room to accommodate expansion, it can sometimes leak out of the cap onto the tank and out of the vapor hose on the ceiling of the tank onto the ground behind the right foot peg. That can be a nuisance, a fire hazard or both. In the States, and maybe in some other counties, there is a flapper valve at the bottom of a short funnel attached to the fuel cap. If used correctly, that prevents overfilling. If your tank doesn't have one of those, you can fill the tank right up to its opening with gasoline and find out what happens while riding, or keep the fuel level a centimeter or two below the cap and see if that's okay.
Limiting filling to 10 or 15 liters seems too cautious. You'll find out. Consider it practice in implementing the scientific method.
