It's much easier to trade a player during the early-ish offseason, when most rosters are not set yet and the teams have more cap flexibility. If the Jazz truly planned to trade Sexton soon they would have attempted to do it a month ago and there would have been be some rumors of him being on the trading block. It is still possible that the Jazz are open to trading Sexton for the right price but they are almost certainly not planning to do it now, before they can see how well Keyonte and Collier are doing.Maybe this is a sign that Sextons on the way out.
The simplest explanation is that the Jazz replaced one veteran PG, Dunn, with another in Mills simply because Mills could be had for less money and years.