They might not need it to draw people back once the park reopens, but they'll need it to help spread out the crowds. Let's say the park opens at 25% capacity, no point holding back SLoP when it's ready to go and can help spread people out and make overall wait times shorter. If SNW is ready by late 2021/early 2022, no point in holding that back either. Opening a new land will add additional capacity and added sales from F&B, merch and Power Up Bands.
I don't understand the need to hold back a ready to open an attraction. This isn't a blockbuster theatrical release that may be competing with other movies so delaying a proper release date is more beneficial in that scenario. RotR opened in December at DHS and January at DL so no reason SNW couldn't do the same if ready.