This is a bit complicated and I'm going to do a bad job of explaining it.
The argument is that there are population clines, which are biological, and then those population clines are divided into races by choosing to ignore or use specific traits at a certain point along the cline. This choice isn't biological, which makes race a social construct.
The underlying population clines aren't always along the same gradient. Skin colour and physical size might be gradients that run a different way across a continent; skin colour might change north to south to north again due to sun intensity, while physical size might change based on altitude and terrain. There's no disagreement that both skin colour and size are partly biological, but the decision to use one over the other to differentiate races is social.
All the things that differentiate race are genetic, sure, but which things are used to categorize people into race is socially constructed. Why do people base racial categories on skin color but not, say, finger length or whether you can wiggle your ears? Biology can't tell you that.
