That's completely wrong. Fingers give a legit boost to Sukuna's CE levels and CE output while getting Megumi's CT is more about increasing his versatility.
It doesn't boost CE Output, it boosts his entire CE reserves. If he has more reserves then he can output more for a longer period of time.
Mahoraga alone was canonically stronger than 3 Finger Sukuna, who himself admitted he cannot beat it.
When you combine that along with all the other Shikigami & power that Ten Shadows brings Sukuna he is logically stronger.
He gets all the benefits of the Ten Shikigamis along with their abilities.
You give Kenjaku Megumi's body, Gojo fucks him with Purple. It's all about how much juice you have and how much you exude to execute your moves. In that sense, the current Sukuna's still behind prime Sukuna.
Why does that change how powerful Ten Shadows is?
Every CT is as powerful as the user itself.
Sukuna already has a busted CT and he's getting another power whose best technique alone is worth 3 Fingers, now Sukuna can control it.
20 Finger Sukuna may have more Cursed Energy than 19 Finger Sukuna, he's 100% not stronger.
Prime Sukuna had his refined body in tip-top shape and his has fingers with no one else sharing his power like Yuji. He had more cursed energy in his prime, period.
Yuji isn't sharing anything.
Yuji basically is like a Cursed Tool who has been dipped for months in Sukuna's Cursed Energy.
He doesn't have Sukuna's CE anymore, all he has is his own and his body.