Oh that's quite true!
Isn't it interesting that Betty Boop and Mickey Mouse were the visual inspiration for a comic about a robot superhero?
Anyway, I'm not denying Disney's influence, but I am saying not to count out the Fleisher influence.
Storywise, the inspiration for Tetsuwan Atom was Pinocchio and Superman.
Disney didn't invent the story of Pinocchio but they pretty much defined him in animation.
And, likewise, Fleisher didn't create Superman, but they pretty much defined him in animation (most modern versions of Superman have more in common with the 30s cartoons than the original comics).
Sidenote: Let's not forget Fleisher's Superman cartoons were probably the first true action/adventure animation. And where would anime be without action stories?
Again, he drew from both Disney and Fleisher.
Of course, both Disney (and Ub Iwerks, Disney's top artist), and the Fleisher Bros. modeled their style after Otto Messmer (creator and animator of Felix the Cat), and I would be shocked and amazed if Tezuka wasn't aware of Messmer himself, so Otto Messmer was probably also a role model to Tezuka.
I do wonder if Tezuka was a fan of Winsor McCay, however.