I wouldn't say it's "confirmed" quite yet. Oda could throw in a twist, here. And some of Loki's appearance might be colored by Loki's own interpretation of the Harley, since he thinks he's the Sun God. It'd make sense if Loki thought the Tree-Dragon was Nika, since it's the one who's closest to the sun. So, some of Loki's own biases may be coloring his form. I know the Giants already pointed out that they know that Luffy was "dressed" like Nika. But, much like the different interpretations of what NIka is supposed to accomplish, I could see there being debate among the Giants about what Nika actually looks like, and which figure he actually is on the Mural.
But, it does feel like it's being strongly implied. Loki definitely LOOKS like the Tree-Dragon, who most likely is the Forest God in the Mural. Which is bad news for me, personally, and my theories. Because I was thinking it would make the most sense if Imu was the Forest God. My interpretation was that the "sending forth devils" line meant the Forest God created the Devil Fruits. Which kind of works for Imu, given the Gorosei and the God's Knights.
Does that also work for Loki? Does it make sense for Loki's Fruit to be the Forest God Fruit, if we assume that also means Loki can create Devil Fruit powers? Well, we haven't really seen Loki empower anyone, yet. And he has had opportunities, since he seems to have those masked followers in the Underworld Realm who were helping him out. I guess you could say that maybe it was Loki who was giving random wild animals Devil Fruit powers, since he has a pretty strong connection to his animal friends. Though...he didn't seem to grant powers to ALL his animal friends, since they didn't seem to be able to fight back very much against Shamrock and Gunko. So, if Loki CAN create Devil Fruits or Devil Fruit-like powers, he's being VERY picky about who he gives them to. But, it's possible he doesn't actually KNOW he can do that. Because, again, Loki thinks he's the Sun God, not the Forest God. If Loki realizes that he's the Forest God, he might get access to his "true" powers.
So, we'll see. My bet before seeing Loki's Devil Fruit reveal was that he was actually the Earth God. Because the same "arrow" pattern that you can see on the Hellflame Serpent, and one of the little "moons" above, is also on the "Ancient Giant" holding the spear who looks like Loki on the left of the Mural. It feels like Oda was hinting at a connection between Loki and the Earth God, there. But, Loki's Dragon form DOES look a lot like the Tree-Dragon, I'll admit that. I'm just not sure how much of that could be Loki MAKING himself look like that, because he thinks he SHOULD look like that. God-Powers might be highly influenced by your own creativity and psychology, after all.
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