And yes all Exynos 5420 devices will get upgraded.
I'd never use the words
all or
definitely in anything having to do with Samsung. The N10.1-14, Pro's, and S's are all using near identical h/w yet all are running different versions of TW with different feature sets and varying versions of Samsung's s/w (EG: My Files, S Note, etc.) Lately, updates seem to be focused on the OS rather than bringing newer or more current functionality to older devices. The Note II didn't get any of the Note 3's enhancements, the N10.1 any of the N10.1-14's, and the N3 didn't get any of the SGS5's; even though they are all running 4.4.2. Hell, Samsung's rolling the updates they do provide out at a snail's pace with the N10.1 and N10.1-14's 4.4.2 update still missing from many markets even though the initial release was months ago.
The N12 was most likely always intended to be a niche device. All the market analysis says 8-9" tablets are now the sweet spot going forward with sales slowing down considerably across all sized tablets based on phablets being bought (especially in Asia) as alternatives. Samsung's tablet strategy over the past year has been bizarre. The N10.1-14 was announced and shipped in November of last year without nary a leak of its impending arrival. Then the Pro's were announced with much fanfare and the less than four month old N10.1-14 never mentioned again by Samsung. Then four months after that the S' orphaned the Pro's.
So it doesn't seem Samsung's tablet sales strategy is that solid and now between Wi-Fi, 3G, and LTE versions of multiple 8ish", 10.1" and 12.2" tablets all running different versions of s/w they've created a quagmire when it comes to updates; especially considering each of those tablets is also running different s/w across God knows how many regions each.
It would certainly make sense based on its selling price that the N12 would be a front runner for updates but its sales volume is well below other newish Samsung tablets. Samsung's logic appears random (did anyone expect the over two-year old N10.1 to receive 4.4.2?) hence avoiding terms like all and definitely.