It sounds like the beta program was like a field test just looking for major flaws in operation or performance. I speculate this was due to the drama in the HE560 rollout. Hifiman obviously didn't want to repeat that experience. And of course, they didn't want an Ultrasone ED10 debacle. (No viral videos of Tyll sickened by a production phone will be forthcoming now.)
So from that perspective, I can see that the beta test makes sense. It was a last minute pause before showtime, and that makes perfect sense to me.
It was not an attempt to make sure 100% of headphiles testified that this was the ultimate headphone (or even a worthwhile one), an impossible quest anyway.
(Again, all just speculation here of course...)