I'm not sure if I'm reading this correctly. The statements below are my understanding, but it'd be great if you can confirm to provide more color.

The pre-patch setup would just make the implicit trust policy explicit, meaning any user or role in the account with `sts:AssumeRole` on `*` could assume the role (which is still the default when not trust policy is specified).

This change improves the posture by adding a trust policy to the role that prevents any roles other than those two listed from assuming the role. So this is purely a defense in depth measure, and not really a security vulnerability (unless we say the default, implicit trust policy is a security vulnerability itself :P).