I doubt it's going to be for one game...
Sure, it
might be used for more games, but if you know how costly engines are, Square-Enix is king at throwing money into making engines that are very rarely used. Look at Crystal Tools; only 5 games released use that engine. All of that money put into an engine that really only shined for Final Fantasy XIII and apparently Dragon Quest X. Even
then they made an engine that had to be worked around which had technical issues that could be plainly seen in Final Fantasy XIV's first release, and Luminous appears to be the same shit all over again with Kingdom Hearts III falling into hurdles with the engine.
I think it's a pretty big deal because Luminous was supposed to be the engine to do everything they wanted that made Crystal Tools suboptimal to work with. Instead Luminous is yet another engine from this company that fails doing on what they want it to do for their games, but they're smart enough now to use external engines to make up for their shortcomings. It's a significant waste of money and resources, something Square-Enix seems to be a pioneer at. If there's one thing they know how to do better than anyone else, it's to bleed money.