It reads to me like it's specifically checking for a 2D texture instead of "not a cube texture", but the distinction may be somewhat moot: WINED3D_CKEY_SRC_BLT implies ddraw, and that means we either have a cube texture or a 2D texture.
Yeah, it could be unnecessarily defensive coding as well. It's not really obvious to me why cube textures or 2D textures would matter for the purposes of GL, though. [I don't suppose you, or someone else more knowledgeable about early GL, has an idea?]
Note that constructs like
<snip> can often be rewritten as <snip> without readability being reduced.
True. I might do that in a separate patch just so the diff is clear.