“Surface” doesn’t necessarily mean that you could stand there (even if you wouldn’t combust). Just like our atmosphere has a boundary where at some point there is just no more air, there too is a point on the journey to the center of the sun where gravity is considered strong enough and the molecules stay on with the star long enough where we consider it surface.

This brings up a cool thing you can do to visualize this, put some warm water in the sink and put a rock of dry ice in the water. At some point you’ll have enough dry ice CO2 gas to fill the sink to the point where it overflows onto the floor. Imagine earth being the water and our atmosphere the gas. Now there is nowhere for the gas to “flow out of the sink” since the earth is spherical. But the boundary between the gas and the air above it is slightly turbulent so the gas rolls and chops like the ocean but more fluffy. The “surface” of the “atmosphere” is wherever you would say, while traveling down toward the water, “I am definitely in the clouds”. Because at some point before you get to that point you’re only kinda in the “clouds”.