I doubt shader aa in a game released in 1999
Edit: the game was later patched to support dx9 so it's possible
I found this (but dont really understand it)
Maskable samples just mean that the samples are "explicit" instead of implicit. And if maskable samples are supported, you can actually use a MASK to write ONLY to those samples (it's not possible for ALL AA modes that's why you need this to be supported explicitely).
The way it works is like this :
each pixel, the hardware computes a mask from the geometry (full mask if the pixel is inside the triangle, half the mask if the pixel is half insider the triangle and half outside the triangle). Then this mask is ANDed with the mask that you specified as a render state. If you selected the 4X_Maskable that means that the mask contains exactly 4 bits. That way you can redirect rendering to only one fourth of the final samples.