One must remember that warmtone papers not only give a richer sepia colour than neutral or cool tone papers, they also bleach much quicker and so, for a given bleach time, the toning may extend further down the tonal scale in the print and so can look more obvious.
However, it is always possible to split sepia tone with any paper if you dilute the bleach enough and time the bleach carefully. I frequently split sepia tone MGWT with no problem.
It is true that Selenium will also give brown or red-brown with many warmtone papers, but the effect is quite different, especially with split toning. Sepia, through the bleach, works from the highlights down and selenium works from the shadows up. So split toning with sepia will give warm light tones on cooler (depending on the paper how cool or warm) lower tones. Selenium with a warmtone paper will give warm brown low values and the normal tone for that paper/developer in the upper tones.
There is no reason why one should not do both on the same print of course.
Tim