They're trading Pettersson during the summer. That's what makes the most sense.
His NMC doesn't kick in until July 1st, and the market for him will be much stronger when teams have cap flexibility. Trading him mid-season, they were going to take a massive haircut.
The difference with JT Miller is that the offers weren't going to improve in the summer because there is no way to create a market for him with his NMC already active. So it made sense to resolve it by moving him now, and then return to the table with Pettersson trades at the draft and before July 1st.