The inbuilt support in lumberyard, according the bumph, looks to be mostly in hooking into Amazon's Gamelift API, which talks to AWS, makes it easier to set up servers, handles some client authorisation, messaging. Without going on a technical dive I can't say more specifically.
Even if you're not using (or end up using) an interface you're provided or are evaluating, sometimes that can give you pointers as you roll your own, so from a coding standpoint, yeah, it's still useful to have a point of reference, especially if the license provides source code so you get a peek under the hood. Sometimes the API level interface ends up being used under a wrapper interface which may not resemble the API much if at all. That's down to the call of the lead in that department and if they think the overhead in doing so is worthwhile.
SC is going to need a great number of server processes, and those processes (due to setup/teardown time as well as propogation of authority data) are going to have to be spun up (likely in response to regional load), reused and balanced (in response to actual game instance requirements) on the fly, otherwise the proposed dynamic instance range mechanism simply can't work erm, dynamically, and you're left with something that's much more like the traditional MMO method of rather fixed zone (or player group) instance processes governed by a central set of gateway and database servers.
An API that deals with the management of instances is extremely useful, yes. Doesn't do everything required, far from it, but it likely helps.
Again, it's no magic solution. It's not a means of avoiding a frankly horrifically complicated setup. It's not a 'yay this will work 'cos Amazon'. It's a set of tools which, to me, look useful.
