I agree. It was Mo asking CS in January what is going on, does he just want to be friends, did he get her texts, etc. Mo was clearly confused and wanted to talk it out. Because she assumed they had something. IMO. Mo. probably felt misled. If Mo didn’t want to be involved with CS she would have been relieved he was there with his GF and wouldn’t have sent that text asking for clarification. And his response, “this was a mistake” - I wonder if he meant his involvement with Mo was a mistake or bringing KA to that January event was a mistake?
I think he was trying to placate MW by saying that it was a mistake to have had KA accompany him to the event under discussion. He was probably telling an upset KA something similar.