I'm guessing they are right. Arabian show saddles, and many regular Arabian saddles, are short, have a rounded skirt, low cantle with plenty of slope, and a sharp rise in the front to the swells. See here: Arabian Saddles
Inside, they have an Arabian tree, which it is impossible to know about by just looking at the outside. The tree is the frame of the saddle, and Arabian trees are shorter in length, usually have more rock (curve between front and back), and are flatter (when looked at from the front, they form a flatter inverted V), but the bars are not set as far apart as on most quarter horse trees. More info here: Western Saddle Fitting and Different Tree Sizes
That should make it fit most Arabian backs better, but it depends on the Arabian horse and on the saddle-maker. There is no guarantee it will fit any individual Arabian horse. And unless the saddle is marked, it is very difficult to figure out if it is using an Arabian tree.