The thing is your power source doesn't make you exclusively one thing. Red mage uses two already known sources, we're fine. Geomancer as per lore uses directional and the elements. Obviously conjurer has access to elements but due to 1.0/2.0 lore/gameplay changes they dont use the full spectrum (and their method of manipulation doesn't need to be the only one, like how primal keep gaining variations on how they exist).
Full spectrum + directional is enough to not be conjurer, like two sources of magic is enough for red mage to stand on their own (not even adding the fact that they could give Geomancer more things to further differentiate, like Red Mage). Or to make a real life example both a rocket scientist and a bridge designer use physics but the end results are massively different (similar origins does not necessitate similar ends).
These lore excuses to place a firm no are flimsy in reality, like with how people said summoner can't have bigger egi because lore but now we have demi (as they added lore to make it happen). All it could take is adding some more bits that expand Geo in a desirable direction for them. Or simply all they have to do if they don't like Hingan lore of geomancer is introduce a geomancer from another area even.
I'm a bit sad that something so pliable (lore) is used like some set in stone can never be added to code of rules. I think fairly recently I saw some of the similar can never be done because lore used for Tonberry and the beastmen system but funnily enough now we have dwarves in that system.
Lore is only a problem when lacking the desire and or creativity to make a solution (lore found a way to make a Goobbue fly, I think enough is said lol). Geomancer is not literally "just" a conjurer, comparing how it plays to it's past iterations they are not the same which is one of the most important comparisons. Is the gameplay the same or not is priority question, not if they use similar colors (Red Mage technically throws some stone around too, that wasn't an issue). They use elements sure but many jobs in FF share some sort of power source or a few spell concepts (things that can be expanded upon in new lore, like masters at wind and water because they're at a port city and around the sea (ty for the point @Lauront) but in actuality they can/do use "blah"). The more important issue is if something can add gameplay value, and of course Geomancer can be- it may need more lore added to it but so does any new job added to the game or when new levels are added at expansion.