How? It contradicts a lot of what Wan did in Aquaman.
Mera's dad is alive and a main character in Aquaman, he's dead in Justice League.
Vulko is different, he never trained Arthur in the Justice League timeline; but he trained Arthur in Aquaman. Vulko's armor also wasn't like Atlantis armor from Aquaman, it was more like Xebel (Mera's people).
They can't talk under water, just clicks and dolphin noises. No air bubbles needed to talk in Aquaman.
Atlantis (where the Mother Box was kept) also looks in ruins in Justice League, when it was neon and futuristic in Aquaman.
Honestly the Atlantis stuff is really the ONLY thing that clashes with the rest of the DCEU. If that stuff was edited down there's no reason it couldn't still be canon.
I didn't hear Mera's father was dead, when did she say that?
It was never said that he didn't train Arthur in Justice League. They only talked about him not wanting his mother's trident or being King, so it could be left to interpretation on any history they had as Arthur knew Vulko's name.
I'll give you that one.
They did attempt to explain that away by saying where the mother box is, was just a strong hold. Weak but still attempted.
They also mentioned that Orm talks of war with the surface (but I don't think Arthur even knew Orm existed in Aquaman, so I'll give you that too)
It explains why Arthur was in some Norwegian mountain town in JL but in a shore town in Aquaman
It explains a little that Mera and Vulko had been trying to convince Arthur to become King for a while as he spoke to them with familiarity. I found it weird in Aquaman that he spoke like he really knew her when in the Whedon cut, he barely spoke to her, and Vulko wasn't in it.