Before the modern times, such huge armies were logistically difficult. Not to mention, how many elves are there in general? Typically elves are depicted as low in numbers because they are immortal or at least have a very, very long lifespan. 300k elven soldiers in a single castle (with more elsewhere) just sounds so wrong. For comparison, 300k is more soldiers than the Romen empire ever had at the same time, in all of its legions combined. It wouldn't surprise me if the Empire of China of old had more, but it feels weird to think of elves to be so numerous.
I have said it before, but the witless numbers in this arc remind me of xianxia. Maybe the author is a fan of Chinese fiction.
For real world, yes. This is a world of magic, they have magic airships and God knows what kind of logistics they can pull. Unless there are more information, just consider it a case of "magic handled logistics".
And yes, China had more.
Battle of Changping, which is famous for its subsequent
massacre, saw 400k Zhao POWs killed, either beheaded or buried alive. Qin launched 2 nearly continuous invasions into Chu, the first one was 200k and the second was 600k. Now, you can say 2/3 of that number were coolies to carry supplies, but the fact that they were able to launch such massive armies stands. While nowhere near a million, it showed the capability of organizing and maneuvering such forces wasn't unheard of in the Sinosphere.
Btw, huge numbers isn't a recent thing. Romance of Three Kingdoms has massive armies that completely dwarfed their real life counterparts. In Battle of Chibi, Cao Cao had realistically 2-300k at best, but in the novel, he got a whopping 800k combatants (or lost 800k in the battle, I forgot). Making numbers bigger serve 2 main purposes: To show how epic the fights are and to flex the commanders' skills as they could mobilize and command such armies with ease, which means they must be exceptionally talented.
To be honest, pre-industry European wars simply did not operate on the same level as of East Asia.