Here's the glossy promo video for Samsung DeX:
In short, DeX is a way of bringing out tweaked Android applications (stock, Samsung and Microsoft/Adobe-created, mainly) into a multi-windowing interface that started being allowed in Android 7. It's nicely done and the multiple windows certainly show where Microsoft's Continuum could and should go on Windows 10.
However, there's a fundamental difference. These are 'just' Android applications, however large they're made on an external display, they're not desktop class and they're not the same as similarly named versions on the desktop/laptop.
The Universal Windows Platform (UWP) applications that run under Continuum are exactly the same applications as run on the phone screen or on tablet, desktop/laptop, XBox or Hololens (etc.) For big named apps like Word, Excel, even Chrome (here), it makes little difference, admittedly, i.e. Word Mobile on W10M and on Android is going to work much the same. But when you venture outside the world of in-box applications and into third party apps then the difference is vast. For once, Windows 10 Mobile and its ecosystem of slick UWP applications is ahead of Android - you can use Feedlab or Enpass or Formula 1/PitlaneOne, etc. and they all install and work perfectly on phone, on Continuum display AND, crucially, on the desktop, exactly as-is.
Perhaps such a distinction is beyond the grasp of the man in the street - and they might only care about a handful of apps when 'docked'. But it's a distinction that's well worth making.
Other differences are that the Samsung Galaxy S8 screen is unavailable when docked - the phone can only drive one display at a time, while Continuum on Windows 10 Mobile can drive the external display while being used independently via the phone's touchscreen. Plus you can use the latter as a mouse for the former. Then add in that the Microsoft Display Dock is quite a bit less expensive.
Against all this is the unassailable fact that Microsoft has stopped making both the Continuum-capable phones and also its dock, so you have to source them from sometimes obscure retailers - Continuum is still part of the HP Elite x3 and Alcatal IDOL 4S ('with Windows 10') worlds, but they're not exactly mainstream. While the Galaxy S8 will, if I know Samsung, be marketed to the nth degree across the world.
So think of DeX as being halfway to what Continuum could be. But expect it to sell better overall!