The latter is true. Phone needs to be in BFU to work against cellbrite, I figure. Lockdown only turns off biometrics and makes the phone unlockable with a pin or password instead, iirc.
From the GOS forums, it looks like as long as you keep your phone up-to-date, block USB data in the locked state, and the phone is in the before-first-unlock state, cellbrite still can't break into it
If the AI was as good as they claim it is, they wouldn't need to pay influencers to promote it.