Making it even more curious, this actually lines up with two separate things - one official, one unofficial;
1: The production of Super Mario Odyssey, which, though only mentioned as the "next 3D Mario game", was stated in an IGN
interview to have also been started in April of 2014, which was the same month that the "Smash 6" job-listing went live.
2: The second part of the Diddy Kong Racing 2 rumours, which circulated from
this source in mid-2014 and again in mid-2015, claimed that Wii U dev-kits were being used to do early work on the development of games for the new hardware that we later learned was the Switch. Curiously, this also somewhat lines up with how the "Smash 6" job-listing lined up with the first half of these rumours, which claimed that the alleged Diddy Kong Racing 2 was originally due for release in 2015 (although that game was still supposed to be for the Wii U at that point).
Also, to add to it, Smash 3DS/Wii U's engine is designed for scaling between wildly differing architectures, so it's probably been designed to be easily ported - after all, they had it up and running on both the ARM-based 3DS and the PowerPC-based Wii U simultaneously, and both games were maintained simultaneously throughout their support-period. This scalability would definitely be a boon on the Switch, since it needs to effectively do the same thing when docked or undocked (it's why I've speculated for most of the time that I've been here that Smash 3DS/Wii U were, in part, a field-test for the then-future hybrid console; I still stand by that
).
If I've correctly understood the information that we have access to, then I'd say that E3 is "now or never" for seeing "Smash 6" - any later and it would suggest that the job-listing wasn't for what it says it was for, since at this point, it's already had more development time than any other Smash title ever made, to date.
They've said it'll be a big E3, they need it to be a big E3, and they need all the show-stealers that they can show. I think that Smash would definitely qualify as one of those show-stealers.
This would be totally in line with how they're working to build buzz for releases on the Switch, really.
I could, too; Reason being, the game looks like it was designed around having both a home version and an arcade one, because it was immediately integrated into The Pokemon Company International's annual schedule of events even though the Wii U was already dead by the time Pokken Tournament was released. Most of the people they want to be competing at Pokken won't have access to the arcade version, after all!
I reckon so, too. Remember, also, that Smash 3DS/Wii U were the only games since Smash 64 to have been created without a fan-poll beforehand; Though they were never available internationally, the Smash 3DS/Wii U ballot had Japan-only forerunners for both Melee and Brawl, and they did say that the most recent ballot would be used for future titles as well.
This always added to my suspicion about Smash 3DS/Wii U being rushed out, incidentally - a content-light Smash game without a poll beforehand was something unthinkable until they were released.