You’re right.

The trays had a groove for a CD. So placing it in the groove it would work because its edges would always fall into that groove correctly all the way around the tray.

This, however, wouldn’t work on a slot loaded drive since they worked by having a set of arms with rollers that grab the edge of inserted disc and another arm with a roller that pushed it the rest of the way in from the opposite edge when it’s inserted enough.

You can see how this worked here on a DVD drive that uses the same setup. https://youtu.be/qi3v7X6BpAA

So there’s only ever 3 slim points of contact which is fine as long as it’s a circle. Yet the irregular shape here would cause it to get partially in and then pushed by the arms into the edge/internals of the drive.