Correct. There was a very complicated and delicate armature inside the drive that guided mini DVDs to the center. The revised Wii had a tray-loading drive, and no GameCube compatibility. So even though you could insert GameCube discs without issue, they wouldn’t play.
Those original Wiis still could not handle the Diddy Kong Racing disc due to the non-circular shape.
I didn’t realize there was an in-between model. So that’s what that black Wii was!
You inserted a GC disc and it didn’t jam? If a mini DVD went in properly and could be ejected, then those guides for the smaller discs were still there, just the software no longer registered the disc as a game.