I think it ultimately goes to what is shown by the preponderance of the evidence? Phone linked to GZ. Receivers testify to getting his texts. Another path might be linguistic analysis to show if the style is consistent disproving the contention that a five year old cracked the phone and texted about fighting etc.
The dog did not bark.
If someone grabbed TM’s phone and started texting to TM’s contacts, the contacts would catch on and there would be a record of these people pushing back and questioning the authenticity of TM’s messages.
I wish West had gotten this in and then asked the expert if there were any kerfuffles over identity of the texting person.