This is primarily because the armor worn by this king appears to be the same suit worn by Arthur Curry in the modern day battle against Steppenwolf’s forces.
This social media post doesn’t provide much narrative context for how Julian Lewis Jones’ Atlantean king will factor into Justice League’s story.
If that is the case, then it is entirely possible that we may also see fan-favorite Wonder Woman Amazons like Antiope and Queen Hippolyta alongside Jones’ character on the battlefield.
However, it seems reasonable to assume that this could be a flashback sequence showing an earlier skirmish involving Atlantis and Themyscira facing off against Steppenwolf’s Parademons during the arrival of the Mother Boxes on Earth.