Is it in the same genus? Yes. No one’s arguing that.
As someone who is a scientist who studies lions, I am telling you, specifically, in science, no one calls lions panthers. If you want to be “specific” like you said, then you shouldn’t either. They’re not the same thing.
If you’re saying “panther” you’re referring to the taxonomic grouping of Panthera, which includes things from tigers to leopards to jaguars.
So your reasoning for calling a lion a panther is because random people “call the roaring ones panthers?” Let’s get snow leopards in there, then, too.
Also, calling someone a human or an ape? It’s not one or the other, that’s not how taxonomy works. They’re both. A panther is a panther and a member of the panthera genus. But that’s not what you said. You said a lion is a panther, which is not true unless you’re okay with calling all members of the panthera genus panthers, which means you’d call tigers and jaguars, and other cats panthers, too. Which you said you don’t.