Their jabs and dash attacks have different utilities. Zelda has a much more rewarding edgeguard with ledge Dtilt and offstage Dair (even sourspot), which is complemented by her better aerial mobility. SoloRosa's Usmash is actually risky save for specific anti-air/OoS punish situations, in which case it's not that much better than Zelda's Usmash (that has the bonus of 25 active frames) or even Farore - and both have way more kill power.Rosalina sans luma still has a better jab, grab and dash attack (due to actually having mobility) that allows her to at least be threatening. She can pressure people off-stage more than zelda with down air and back air, can juggle people better with up-air, and has a more reliable kill option in up-smash.
Zelda is actually an awful character that can't compete, rosalina sans luma still has basic options that work, they're just way undertuned.
With that said, Zelda also has better damage, punishes, overall kill-power, OoS options, combos, mixups, offense and ways of getting back onstage.
My point is not to oversell Zelda as your generic "character people sleep on", but claiming she's worse than Solo Rosalina is a bit of an exaggeration. Luma IS a big deal for Rosalina and there is a reason Dabuz plays conservatively with air-dodges and rolls most of the time he is Lumaless.