Therefore, as of 12:01 a.m. PT today, SAG-AFTRA is on strike against the following video game employers with regard to all games that went into production after Feb. 17, 2015: ​

Maybe Des2ny got started before this date.

I think you answered your own question. I mean, I'd be pretty surprised if SOME work hadn't started at that point.

Therefore, as of 12:01 a.m. PT today, SAG-AFTRA is on strike against the following video game employers with regard to all games that went into production after Feb. 17, 2015: ​

Maybe Des2ny got started before this date.

It's also possible that the contract is between Bungie and the voice actors instead of through Activision. This happened quite a bit in the TV strike. Like when Letterman never stopped airing newly written episodes even though the writer's strike was going on because Letterman's company had already agreed to the new terms, even though CBS did NOT agree to the new terms. Bungie and Destiny could potentially be in a similar place where Bungie was already properly treating the voice actors and so the voice actors could still work for them.

Voice work completed prior to the strike/contract obligations?

Those would be what I'd spitball as probably causes.

I would find it incredibly hard to believe that Bungie had all the dialogue recorded a year prior to release.

My only guess is:

Therefore, as of 12:01 a.m. PT today, SAG-AFTRA is on strike against the following video game employers with regard to all games that went into production after Feb. 17, 2015: ​

Maybe Des2ny got started before this date.

^

As far back as I could tell, Destiny 2 seemed to get started upon in March 2014?
(yes, prior to the wide spread proliferation of HDMI 2.0a and the HDR/HDR10 open standards).

A small group of the Destiny 1 team broke off from Destiny 1 and began on Destiny 2?

http://www.rollingstone.com/glixel/interviews/destiny-2-lead-luke-smith-we-want-to-unhide-the-fun-of-destiny-w482878

"JD: What was your role on Destiny 1 versus Destiny 2?

LS: My first job on Destiny was as the lead designer for the raids, and so that involved finding a bunch of talented folks who could join Bungie and become the raid team. Then we worked together on what raids would be in a first-person shooter, and we built a bunch of prototypes. Ultimately we built the Vault of Glass, which was Destiny's first raid, and basically while we were finishing that, in March of 2014, I was also working on the game that ultimately became Destiny 2.

"JD: How close is what you've done to that original scenario that you thought of back in 2014? Is it pretty close?

LS: Yeah, it is. It's the scenario that we sat down and wrote. That's Destiny 2.

As for when it truly went into "production", that is another question we might not be able to answer as easily.
The Taken King has to fit into the timeline somewhere, which Luke Smith was involved in. And there is also the rumored delay that Destiny 2 was subjected to.
If Bungie has the proper paperwork, I'd imagine they could prove that some type of production on the game Destiny 2 began prior to Feb. 17, 2015 - as far back as March 2014?
(We don't even know, maybe the unions 'production' requirement was at least one line recorded by one eligible voice actor in the Voice Actor's Union?)(I believe some lines from Destiny 1 are featured in Destiny 2 - or at least exist in the D2 file store? These D1 lines were recorded prior to Feb. 2015 and in some sort of loophole might even be enough to qualify the game Destiny 2 as being in production prior to the cut off date? Who really knows?)