he's early 20th century, but not a 'modern' [at the time] composer.
Niko is by far one of my favorite composers and I'm assigned several of his works currently by my teacher and I see where he can lean back and look over his shoulder back towards romanticism, however he showcase his ingenuity with a command of very complex harmony, which looks forward at shifting aesthetics and practices and traditions that really push tonality to the fringes.
Liszt was a 'romantic' composer, but if you listen to his very late works, he had glimpses of what would be impressionist tenancies and he stretched the limits of traditional tonality at times, but we would not call him impressionist.
Scriabin was an early 20th century composer but his early style is much more romanticized until we get past about op 50ish or so....
Hummel was a classical composer but he bridged and did things that leaned more towards what early romanticism would flesh out to a greater degree.
Elinor Remmick Warren was a modern composer but more of a neoromantic and closer aligned to John La Montaine, Aaron Copeland, Nicolas Flagello, than say any number of forgotten 'new aestheticists'
Niko was genius and probably the greatest 20th century composer that mainstream classical enthusiasts either don't know about or [unjustly] ignore.
When I discuss him I generally say he was an early 20th century neoromatnic.
(similar to how mid 20th century we had a LOT of mid 20th century neoclassical composers), ie Jean Francaix, etc.