I cannot speak to the author's intent, (and if anyone can, via author interviews or databooks, I would encourage them to write an answer.) however, there are in-universe reasons to believe Spiral Zetsu is not Tobi (the Akatsuki member).
Herein, to avoid confusion, I will use the name Tobi only to refer to the Akatsuki member with the orange mask. I will use the name Spiral Zetsu and Tobi's real name (spoilers ahead) to refer to those characters.
First off, let's examine Tobi's mask. Though it looks like Spiral Zetsu's face, it is orange. Did he paint it? But when we see Spiral Zetsu show up again, his face is still white. It's shape also does not conform to what we see in Episode 345: I'm in Hell, when we see Obito wear Spiral Zetsu on his body. Spiral Zetsu, being hollow, wraps his whole face around Obito's head. But Tobi's hair is always sticking out when we see him. In the episode, there are three forms:
- A form where Spiral Zetsu's whole face wraps around Obito. We see the spiral mask in this form, and he looks quite like when Obito is pretending to be Tobi, but with a white mask.
- A hood form where Obito's face is exposed. The top of Obito's head is still covered.
- A form where Spiral Zetsu comes off of Obito's head entirely while still wrapping around his body. In this form, Spiral Zetsu's swirly tendril-like head and upper body become shoulder spikes for Obito.
While he's in his Tobi persona, Tobi's hair always sticks out. We don't see this in either forms 1 or 2, and he's missing the shoulder spikes in form 3. Additionally, in Episode 99: The Rampaging Tailed Beast, Toby takes his mask off (yeah, I know it's filler, and y'all think filler isn't canonical, but he takes it off to talk with Kisame too, though I don't feel like looking for that scene). The mask is orange and it is removable. Spiral Zetsu's face does not work like that. Thus, the mask is not Spiral Zetsu's face. Tobi does cover a large portion of his head with cloth, so could it be he's hiding Spiral Zetsu in form 2, just with his head uncovered somehow? I would contend the volume under the cloth doesn't match. Spiral Zetsu looks kind of bulky in form 2. Additionally, when we see Yamato come out of Spiral Zetsu, we again see the shoulder spikes, so it really seems that Spiral Zetsu has not changed. Thus, it is highly probable Obito is not wearing Spiral Zetsu during the series.
Personality wise, I think there are actually some differences between Spiral Zetsu and Obito's Tobi persona. While Tobi's humor comes off as a goofy human, Spiral Zetsu, though also goofy, can't really pass as a human with his humor. He's obsessed with asking about poop since he doesn't need to, and doesn't seem to understand what being a normal human is like.
I also noticed while looking through these episodes that an echo filter is used for Spiral Zetsu's voice, but not for Tobi's.
This answer is not definitive. But if all of this is actually true, and we're going to say that Tobi really is Obito, I think it is also important to acknowledge that in this case, Obito definitely based his Tobi persona on Spiral Zetsu. They were close for years, after all, so the similarity is not a coincidence.