(This is all just speculation, so don't take it to heart)
Good question! I got the feeling form RoW that Mr. T and Dalinar were enemies, and the only thing that I remember Dalinar haveing on Mr. T was his attempted assassination and that did not make them enemies at the time. So I would guess that Szeth has told Dalinar. It has also been a year and so it is very likely that it has come up at some point time.