But Paul says: I met James; and he says: James is the Brother of The Lord. That is what is being discussed. It suggests that Paul believed in the physical humanity of Jesus. Moreover other sources, or at least one other source, tells us that Jesus had a brother of that name, even assuming Matthew picked it up from Mark, and supposing the Josephan reference to be interpolated, which it may well not be.
If you had watched the film of Carrier (as linked above), you will find there Carrier giving an explanation of what "brother of the lord" meant as someone baptised into the faith.
You may or may not accept Carriers explanation. But does give there a perfectly logical explanation of what was meant by "brother of the Lord". Through appropriate baptism, the baptised all became sons of God like Jesus, and hence all brothers of the lord. No doubt if you get a copy of the book then Carrier will explain that in more detail with references.
But either way, as discussed here many hundreds of times, it is by no means clear that Paul thought James was anything other than a "brother" in the faith. And what's more, that same James never claimed to be an actual brother of Jesus. In fact neither James, nor Paul, nor any of them ever claimed even to have met any human Jesus.