To be fair, that was a 5 minute video of what was likely a 60+ minute work-out. I've seen plenty of guys go at that speed for long periods of time during one-on-one work-outs that were designed to build muscle-memory and improve technique. Most of the times when I see guys coming in before practice to work on specific stuff, it's more about repetition and building a good habit. I don't doubt that he has also had work-outs this summer that were a little more ramped up. Pretty sure there are plenty of NBA and wanna-be NBA guys who have a significant amount of their individual work-out time that goes at about the same pace.
The first step in being able to make 3's at NBA range is to make a lot of 3's at NBA range. Then you gradually start taking 3's with a defender nearby. Then you work on making challenged 3's with guys running at you (which I doubt many coaches encourage their big men to do in a game unless there nobody else open and the clock is running down). Seems like when I see bigs shooting 3's in the NBA, it's off a Pick n Pop designed to give them a nice, clear look over a guard or nobody, or when the defensive big doesn't bother to come out to guard them.
He doesn't have a great long-range release, probably the first time in life he's even had to worry about making 24 foot shots, but there are a lot of guys who came into the league and couldn't shoot the NBA 3 consistently, kept working on it and eventually became good enough.