Lawyers will generally exhaust legal defences before moving onto the facts. It's probably Andrew himself is just following advice here. There may be further technicalities come up..
I'm not quite sure what the accusation is here...it's not criminal statutory rape case (and I'm guessing the state of Florida are not sure they would get a conviction, and it would be for them to prosecute this).
They went after Maxwell on criminal charges for trafficking minors across state lines (a federal crime).
She is seeking financial damages in a civil case for him having sex with her when she was below the age of consent. I wonder if in a civil case, if he says the genuinely believed she was 18 at the time, that is any sort of mitigation. One sort of assumes he is not an introspective type that wondered why girls were being "introduced" to him, or thinking to ask how old they were.
It would be very interesting to see if he sweats under deposition. He won't be able to not answer questions, or plead the 4th...and depositions can be very aggressive (I've been through it a few times as an expert in the federal courts). There is no judge there to ensure fair play...and it all gets transcribed. Bits that are objected to will be redacted and not used in the trial.