The iPhone 6+ renders internally using @3x assets at a virtual resolution of 2208×1242 (with 736×414 points), then samples that down for display. The same as using a scaled resolution on a Retina MacBook — it lets them hit an integral multiple for pixel assets while still having e.g. 12 pt text look the same size on the screen.
So, yes, the launch screens need to be that size.
The maths:
The 6, the 5s, the 5, the 4s and the 4 are all 326 pixels per inch, and use @2x assets to stick to the approximately 160 points per inch of all previous devices.
The 6+ is 401 pixels per inch. So it’d hypothetically need roughly @2.46x assets. Instead Apple uses @3x assets and scales the complete output down to about 84% of its natural size.
In practice Apple has decided to go with more like 87%, turning the 1080 into 1242. No doubt that was to find something as close as possible to 84% that still produced integral sizes in both directions — 1242/1080 = 2208/1920 exactly, whereas if you’d turned the 1080 into, say, 1286, you’d somehow need to render 2286.22 pixels vertically to scale well.