What is a genius?
This forum already asked itself this some time ago in a topic and, as always, when similar questions are posed, one never arrives at a shared answer and, therefore, it seems useless to reopen this discussion here, which would result in nothing.
I only permit myself to state that if it were true that there were so many geniuses - or immensities - they would not be such if they were only great composers, or painters, or sculptors, or poets.
The genius is a phenomenon that within the scope of the history of an art presents itself only very rarely! Otherwise, it would not be such!
Beethoven was a genius, Schumann was not! A very great composer but not a genius.
And regarding his phrase, many very great composers, after Beethoven, did not fully or completely understand his artistic Work and others - like Schubert, Berlioz, Schumann - precisely!!! - perceived it, but not to the end and did not know how to draw the due consequences from it.
Beethoven was fully understood only in the 20th century and not by everyone. This is not because they were greater geniuses than those of the 19th century, but because in the meantime - paraphrasing Schubert - a lot of water had indeed passed under the Danube.
Beethoven shouted to his friend and protector Prince Lichnovsky that there was only one Beethoven. And he was right!
As with Alighieri, Buonarroti, or Shakespeare, there is only one Beethoven. They are not the only "immense" ones in their arts. There are also a few others, but for different reasons.