Signal app can update itself at any time
The app is constantly phoning home to Signal servers checking for updates even when it has not been launched and is not being used
That means the client could change at any time, for any reason, unbeknownst to the user
If the advanced user is free to write, edit and compile source code for a Signal client, software developers might call this a "third party client" because there is allegedly some "business transaction" between Signal Corporation and the user where Signal Corporation and the user are first or second parties (although, curiously, the Signal app and service are free)
But it's arguable the more important use of the term "third party" in this context, i.e., "secure" communications, is to indicate a party that is not a first or second party to the communication, a potential eavesdropper
Signal Corporation is a third party to the communication
Because it forces users to use its closed source client software that can be updated at all times for any reasons when it's installed on a user's computer, there exists the potential for remote code execution and, for example, eavesdropping
For example, a US corporation subject to US law could be legally forced to eavesdrop on a particular user. This could be done with an "update"