LETS DO THIS!
XInput enabled in InControl = Only four gamepads work, rumble works.
XInput disabled in InControl = All Five gamepads work, rumble does not work.
I was wrong! haha
1) More than four Xbox One gamepads do NOT work with XInput, exactly as it is with 360 controllers.
2) You and that article were correct that Xbox One gamepads do have triggers on separate axis with DirectInput! Which... makes things more complicated, but it is cool.
Attached below is Windows 10's "Game Controllers" setup dialog window of the inputs it sees with the Xbox One (left) and 360 (right) gamepads. You can see that the Xbox One gamepad has an additional axis (one is left trigger, the other is right trigger).
So... once Swordy supports DirectInput, controls will be unchanged with Xbox One gamepads. We'll still need some alternative layout when using DirectInput with 360 gamepads.
As a side note; I think how I was mistaken with thinking 4+ Xbox One controllers work with XInput was that for a while we were enabling InControl's "Enable XInput" on Awake, which we found out later meant it wasn't actually using XInput. It needs to be enabled prior to running the game. Anyway - during this time is when we did the 4+ player tests!
The problem we have currently have (and previously had) with Swordy running with DirectInput (XInput disabled in InControl) is that we get very weird behavior with button/trigger inputs crossing over between certain players when other certain players press a button/trigger. Example: Player five pulls trigger and holds it down, there is no action on player five's character but when player four presses their bumper, player four's character receives player five's trigger input while pressing that bumper... I would think that this is some bad error on our side if it wasn't for the problem completely disappearing and working exactly as expected when XInput is enabled. So we will have to resolve what is going wrong here to have DirectInput working in Swordy!