that would make the most sense yeah. that way they don't have to waste GPU power for running at higher resolutions that are useless on a small screen anyways.
and it makes it easier to optimise for both docked and portable mode.
if handheld mode targets a native 720p, and the docked mode targets a native 1080p through 4K DLSS Perfomance mode, both modes will look good on their respective screens.
and if really demanding games get ported over, then there's always more agressive DLSS use to fall back on. like using
4K DLSS Ultra Performance (native 720p) in docked mode, and maybe 720p DLSS Quality mode in handheld... they could also try stuff like 1440p DLSS Ultra Performance (native 480p) in handheld mode, which then gets supersampled down to 720p, and other stuff like that.