Thats our whole discussion above , which Z need to know Y private keys, similarly to the TLS attack even mentioned in the link you provided:
Controlling a Client Certficate
Various options for an attacker to control the private key of a client certificate installed at a client device exists and are conceivable. One very natural way that comes to mind would be to employ social engineering technique
So if your private keys leaked its game over not just for Tox but many other tools as well.
Agree, This actually what is annoying about Tox which is its not new protocol been like 7 years but until now not yet audited. Hope they do that soon.