I turned on the chromatic tuner and played some of the notes and good lord, it’s in tune*! No tweaking! 
*or close enough to being in tune, given my still-rookie breath control.
Breath control is what it’s all about.
When you are playing to a digital tuner, you are using breath control to center the pitch. To get a better idea of how “natively” in tune your whistle is, get a friend to watch the tuner while you play.
No wind instrument, from a ten dollar whistle to a golden Boehm-system flute worth more than your car is “automagically” in tune by themselves–they all have to be played in tune by their player.
All of that said, I’ve seen some high D Gens that were well in tune and easy to play in tune. I have seen others where the intonation seemed off.
–James
Yes Generation whistles are funny, you can go through 20 or 30 of them and they’re all different, and you might actually find one or two that play in tune!
I bought a new Generation D a couple years ago that’s near-perfect in voicing and tuning, picked from two dozen. Some of them were just awful.
You can also use the software “flutini.” Run a search on this forum for a link to their website. It samples the notes as you play and gives you a bar graph of how far each note was off over the entire time you played. Like the tuner, it works better to check your “native” tuning if you don’t watch the display while you play.
Wow. You’ll be telling us it plays all the right notes in the right order next. 