It is pretty interesting how open the XCOM 2 team is with modding. Can't think of any other modern day dev that allows modders such total access to their work.
Even Valve, the once-renowned champions of open source gaming now gates everything and tries to turn all their mods into money.
That being said, I'm pretty sure the Long War team isn't just freelance modders any more. 2K have to be giving them something at this point since they even get so much coverage on XCOM 2's own website.
There's a bit of a story on that one, I think. Firaxis X-Com was clearly a labor of love in reaction to the extreme hate of the Bureau's original development, and while it won a lot of people over it was also just supposed to be what it was instead of a basis for anything, since it was a risk. Part of this meant no attention whatsoever was given to modding.
Even to this day, Long War is perhaps the only true mod for X-Com EU/EW. Everything else is altering the ini. files. Remember, Long War is pretty extensive, and almost
all of it was done through hex editing, which is insane. Not to mention that they were unable to introduce any new assets to the game, and had to make use of the ones already there, which is why the SMGs are scaled down assault rifle models and such.
This became such a huge thing that the X-Com development team took notice, and it seems pretty clear that they were deeply effected by this dedication to improving what they made, all the way up to Solomon saying X-Com is just a tutorial for Long War and Firaxis employees providing asset assistance to the mod. It is thus little surprise that when developing X-Com 2, they did everything in their power to open this up for the player base in general and to support the Long War team.
Both sides are almost certainly getting the same thing from one another: free publicity. Firaxis from a business standpoint should clearly allow the Long War team to use them as a platform, because even not allowing that last time resulted in the fame from Long War. The Long War team are planning to make a game of their own eventually, and if it ends up being another UFO invasion game there's little that would hold up that impending Kickstarter better than "from the team that made the Long War and Long War 2 mods". I've seen even OldCom purists become converted and salivate with lust upon being shown Long War 1.