The same logic you are making about Forspoken applies even more to FF, given its pedigree and buzz. And the fact that SE, at the end of the day, knows where its bread is buttered.
And if FF16 comes out with a date by June that's during the holidays, the resulting hype will overshadow whatever work Forspoken has done over the two years or so of reveals and trailers and yadda yadda.
There is also the possibility that they would feel the gap between October and December is enough time for the former to have its time out in the sun.