Alex Donaldson, who runs RPGSite and did all of those old interviews with FF people and the people who worked on Luminous and XIV engine, pretty much confirmed XVI is using XIV's engine if that cgworld article wasn't enough proof already. Interesting thing is that XIV 2.0 / XVI's engine is also forked from Crystal Tools and basically a heavily modified and upgraded version of it, which makes sense given how the Luminous guys who were already busy with Luminous at the time, were able to help the XIV team quickly build another custom engine specifically for XIV 2.0. Would have taken a lot longer if they had tried to do everything from scratch. I imagine the Radec engine trademark is whatever this upgraded CT engine Yoshida's team is using is called now.
"Extremely late reply as not been on Era in days, but my understanding of it from talking to people both in official interviews & beyond was that FF14 1.0 was built out of Crystal Tools just as FF13 was, and then Yoshida's team continued with that and built on it a lot over the years.
In addition, some tech Luminous developed did find its way back into the 14 pipeline. There was a lot of this around the 1.0 time, like I remember playing a 3D version of FF14 at Square's office once - that never became a viable product, but that was Luminous people doing that. It was the same core tech, but they were building on top of it. I know some Luminous lighting/shadow stuff got backported and made its way into 14, too.
Anyway, the 14 fork of that continues on now, really, 'cos they whipped it into shape over many years and then used that tech for 16, which is why 16 isn't UE like FF7R, or Luminous like 15/Forspoken. They wanted to continue on tools they knew inside out, so they did.
The important thing to remember about Crystal Tools in particular is that people call it an engine, but is isn't - as the name suggests, it's just a series of tools. Which in fairness - so is any engine, right - but in those 2010s FF13/14 days, it wasn't formalized into any structure. That is also part of why so much of those developments got so fucked up and complicated. It was just a set of in-house tools that were being used in wildly different ways by wildly different teams. It was hard to share stuff. Whereas Luminous is built in a way where theoretically a bit of training and you could get any team making games in that engine relatively quickly, especially if said games are RPGs or action games with large-scale worlds.
It's actually similar to a lot of the troubles faced when EA tried to get everybody on Frostbite, and BioWare discovered that all the shit that DICE had built was useless for making RPGs, causing all manner of struggles for ME/DA. It's not like a UE or Unity situation - which is what Luminous is like. It's a more old-school/traditional style of game development in that sense. So the lineage from FF13>14>ARR>16 is really weird and convoluted, whereas the lineage from Agni>15>Forspoken is pretty clear-cut, and you can even see how they just render and have the same 'feel' about them."
"One thing I'll say is that most of the core people behind the first iteration of Luminous/Agni's Philosophy are no longer at Square. Most of that core team's leadership was ex-Sega, having done the Hedgehog Engine over there. Everybody I interviewed for Agni's Philosophy all those years ago is no longer at SE. So it's the same tech, but leadership wise, it's all moved on.
As for Crystal Tools - I'm sure SE would argue it no longer really exists. But whatever FF16 is built on, if it even has a formal name, is built atop its foundations, basically."