Why is the Sun's surface a good example of grey atmosphere?

I’m approaching the study of elements of Astrophysics, and the teacher, while speaking about the grey atmosphere model, said that for the Sun’s surface the model is a good approximation of what actually happens.
Why is that so?

I unfortunately wasn’t in class during this lesson and no one of my colleagues seems to have an explanation for this, I also tried documenting myself online but I couldn’t find anything.

I have a feeling that the answer might actually be really simple and I’m just not seeing it for some reason.

Could anyone give me an answer for this?