What makes you believe that RB wont do like almost every F1 team since 1950, including RB, and run the real new RB at day 1 in testing? teams that do otherwise are exceptions.
Do we have a statement or a specific info about this?
Or is this usual broken record/myth/cliché of "the completely new car bla bla" ?
The race-ready car?
Every team runs its race-ready car... in the first race...
That's pretty much been the trend lately.
The trend since Jesus Christ is to run the real car from day 1 in winter testing. So the car that runs in the last day, or in race 1, is 'just' an evolved car with some updates.
The 2022 or 2019 Merc or 2012-2013 RBR are not even exceptions. They just updated their day 1 car more heavily/aggressively than usual.
The exceptions are... I dont even remember (any idea?), so much it's a rare/old practice that a team ""races a completely different car from testing""
Maybe Ferrari in 2003 and 2001, but this is also a different case...
Launches are a different subject.