The corona is not the surface of the Sun. It's the plasma & gases that surrounds the Sun and is the hottest part of the Sun. Which is why this milestone in the Parker Solar Probe mission a big one. The scientists weren't exactly sure if the probe would survive even one trip through the corona. I believe it is going to get progressively closer to the Sun on each pass."15 million miles from the surface of the sun"
There could be many definitions of surface...