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?]
It may not be about GL. One thing worth pointing out about wined3d from that time (see e.g. "git show 4640be8d:dlls/wined3d/wined3d_private.h") is that IWineD3DTextureImpl and IWineD3DCubeTextureImpl were separate structures. It may simply be a matter of colour-keyed cube textures being too obscure to be worth the effort without having an application using them. And that's assuming those are supported by ddraw at all; they may not be, IIRC there were some restrictions on colour-keying.