Transparent depth on model

I found a very clever method for masking an object by Will Olis that uses Transparent Depth.
Works great on part of this model, but the head, pouch, tail don’t go transparent. Any idea why?

Basically what I’m trying to do is add cgi characters to live video. When the character goes behind a chair it should be transparent, then when it goes in front of the chair it can be seen again (the chair is in the live video).
Attached is a simple test with just the model and mask.

kangaMaskTest.blend (1.1 MB)