It can still be flat at this point. After it reaches about shoulder height in the finish, the left wrist is going to want to become bent again. In my opinion, don't worry about this. It's not a big deal in the grand scheme of things - the ball is already long gone.
There will be some who think the left wrist should be flat at the finish position...to each their own. Just don't focus too hard on keeping it flat all the way to the finish, as this will disrupt your swing rhythm by increasing your tension - just let it do what it wants.
But I suppose that the longer into the finish you keep it flat, the more educated your hands are. At least have it flat once it hits the plane on the target side of the ball, as this indicates a good motion and a correct flat left wrist at impact. So if you can keep it flat deep into the followthrough and even further, you know you've had it flat at impact and you get some good feedback.
Really good stuff, Robot Buddy R2D2. You're doing your homework!
A clarifying point:
Make no apologies for the Bent Left Wrist after the Finish Swivel. It isthere for a reason, and if you ignore it, your Stroke -- and your Shot -- willsuffer.
Yes, there is magic in the Flat Left Wrist, but like anything else, that magiccan be overdone. Per the 'Useful Combinations' of Chapter 5, the Wrists move intandem. As you have correctly stated, the Swinger's Backstroke Motion finds theLeft Wrist Flat, Level and Turned (5-B-1) and the Right Wrist Bent,Level and Turned. After the Finish Swivel (2-G), we find the oppositecondition: the Right Wrist Flat, Level, and Rolled and the Left Wrist Bent,Level and Rolled. This 'Golf as a Double Handful' Wrist Combination is notspecifically listed in Chapter 5, but it is correct. For study, the Combinationof 5-D-1 and 5-D-2 is the prelude to the Wrist Combination I've described.
If the player is ignorant of this correct Wrist Combination and overrides it byattempting to maintain a Flat Left Wrist -- when, in fact, it should be Bent-- then Big Trouble is on the way. Even though the Ball is 'long gone'by the time the Flat Left Wrist Override occurs, The Computer (Chapter14) knew it was coming long before, and it will disrupt the ClubheadOvertaking through Impact to accomodate it.
This is a very important point. One, quite frankly, that puzzled me for yearsand that I was quite happy to finally put to bed. Never forget, Precision isrecognizing and reconciling minute differentiations. Sometimes you have to'connect the dots' -- as we did this time -- but, nevertheless, there is ananswer to every Stroke question...