>>281599967
Assuming these similarities are more than superficial, what you're basically suggesting is that this hypothetical DLC superboss and one of the strongest characters in the lore would've mostly lifted moves from generic mooks and the player character and done them worse.
>some of her attacks aren't avoidable by just rolling forward
Only Waterfowl and Scarlet Aeonia cause of the lingering dot. Multiple bosses in ER have moves you can't just roll into so that's not indicative of anything. Waterfowl as others pointed out also didn't even look like that in earlier versions and they likely intended it to stand out with the final version.

Elden Ring has been in dev since at least 2016 and Malenia's design was already finalized by June 2019 (reveal trailer), merely 3 months after Sekiro released. That trailer was outsourced and if we consider production time and marketing schedules, her design is likely much older.
I think it'd be naive to believe that they had no plan for one of the biggest boss fights in the game and had to rely on Sekiro scraps. I'm sure there was some influence since the games were being developed in parallel but there's no evidence to prove that an entire boss was repurposed to make Malenia.