Ok, lets take it step by step:

Thanks for sharing your concerns here. We have been progressing use of our SDK in more use cases for our clients. However, our goal is to make sure that the SDK is used in a way that maintains GPL compatibility.

• the SDK and the client are two separate programs

I think they meant executable here, but that also doesn't matter. If both programs can only be used together and not separate, and one is under GPLv3, then the other needs to be under GPLv3 too.

• code for each program is in separate repositories

How the code is structured doesn't matter, it is about how it is consumed by the end-user, there both programs are delivered together and work together.

• the fact that the two programs communicate using standard protocols does not mean they are one program for purposes of GPLv3

The way those two programs communicate together, doesn't matter, they only work together and not separate from each other. Both need to be under GPLv3

Being able to build the app as you are trying to do here is an issue we plan to resolve and is merely a bug.

Not being able to build a GPLv3 licenses program without a proprietary one, is a build dependency. GPLv3 enforces you to be able to reproduce the code and I am pretty sure that the build tools and dependencies need to be under a GPLv3 compatible license as well.

But all of that still doesn't explain what their goal of introducing the proprietary SDK is. What function will it have in the future? Will open source part be completely independent or not? What features will depend on the close-source part, and which do not? Have they thought about any ethical concerns, that many contributors contributed to their software because it under a GPL license? How are they planning on dealing with the loss of trust, in a project where trust is very important? etc.