Soong was attempting to create the positronic brain, something which nobody in Star Trek canon has been able to replicate (meaning it is exceptionally difficult). Memory Alpha's synopsis of B4 adds some clarity
B-4 was a prototype android constructed by Doctor Noonian Soong on Omicron Theta in the 2330s. He was one of three failed prototypes and the only one to survive a significant amount of time after his activation before the construction of Lore was successful.
Bruce Maddox is working from an entirely different methodology than Soong did. Remembrance made that clear (emphasis mine)
JURATI: No one has ever been able to redevelop the science used to create Data.
Then came Bruce.
JURATI: He recruited me out of Starfleet. Despite Data's death, we came so close.
Then we got shut down, and it crushed him.
PICARD: Where is he now?
JURATI: He disappeared after the ban.
PICARD: I've tried to find him, but You said "despite Data's death", meaning that any new synthetic would have to be made from Data.
JURATI: Advanced ones, yes. If you had Data's neural net, perfecting a flesh-and-blood host body would be relatively simple.
PICARD: But his neurons died with him.
JURATI: See, now you're coming around to that "no" I've been promising you.
PICARD: Does this mean anything to you? [shows her Dahj's double-ring necklace]
JURATI: Where did you get that?
PICARD: From my tea-drinking companion. The one you said couldn't exist.
JURATI: I really really wish you'd come here on my day off. It's a symbol for fractal neuronic cloning.
PICARD: I'm sorry?
JURATI: It was a radical, beautiful idea of Maddox's.
His theory was that Data's entire code, even his memories, could be reconstituted from a single positronic neuron.
If there is a synth out there who is perfect, like you say Then Data, or some part of him --
PICARD: an essence of him --
JURATI: Essence, yes. Would be alive. There'd be no way of knowing - without examining Dahj
PICARD: Uh, I'm sorry? "Them"?
JURATI: They're created in pairs.
The fact that Picard himself was surprised by there being a "pair", means this was new to him as well.