That's not entirely accurate:

https://en.wikipedia.org/wiki/Berghuis_v._Thompkins

Your fifth amendment rights haven't been invoked until you say so. The Miranda warning isn't a guide for how to invoke the rights, just a notification that you have them.

Now, it's ridiculous that we've ended up here and it was a terrible ruling, both of them, but if you're just quiet you haven't invoked your right to silence anymore than you've invoked your right to legal counsel.
Which is a hell of a position for the court to take on coerced confessions.