I would say based on the charges and plea agreement he doesn't have to defend his assertion. Seems to me that's what was agreed upon in the settlement.
How do you propose something OTHER THAN what was agreed upon is true? The case is settled. There is no longer doubt about culpability in this matter. What is it about a "settlement" you don't understand?
The only outstanding question here is why those involved in killing Mr Marshall were not charged with any crime(s).