That seems very wrong just on the face of it. Stockhausen has written numerous papers about his theories of rhythm. And if you listen to his music there is undoubtedly a sense of rhythm, just not any conventional ones. There are also definitely melodies.

Serialism does not mean non-melodic. Procedural creation of music does not mean the melodies and rhythms "don't exist". If anything, they're more there than they are in traditional music.