The sources for the rumour didn't ambiguously suggest a 3rd version, they suggested a Switch version.
Plus, how do you explain this part of the Eurogamer rumour?
You honestly think that if the game discussed in the rumour was just 3DS Ultra Sun/Moon that a release less than a year apart from the original would even be entertained by Nintendo or Game Freak? Please resolve this inconsistency.
There is nothing that indicates their sources were wrong yet, so why print a retraction? And more importantly, perhaps their sources are strong enough to give them ample reason not to.
They didn't make this up by extrapolating from an image, please read the Eurogamer piece again, they were told by inside sources that a Switch version was being made in no uncertain terms.
The sources thought it was Switch. The sources were wrong. These are still leaks and rumours. They can take on inaccuracies. The platform may seem like a massive inaccuracy to us but to some member of staff leaking it, not a big deal.
Yeah, I can believe it would be scheduled for less than a year later. They may be planning Switch Pokemon games like D/P remakes or Gen 8, and they may want a bigger gap between Ultra S/M ('Stars') and the next release on Switch.