This article makes a lot of assumptions that seem to be based on how he thinks things work instead of providing proof of how they work. He states that the keys "seem" to live forever in userspace RAM, yet provides no proof of this.
The code is here. It's pretty explicit. https://android.googlesource.com/platform/system/vold/+/andr... Here's an email from Paul Crowley to me. Paul is one of the devs on this code: https://twitter.com/ciphergoth/status/801220048622714880?lan... Not really sure what else I can offer you.
Also, I want to clarify that I don't think the weaknesses of Android N's encryption are the fault of Paul or anyone else on the team. I think this is really a question of corporate priorities. If Google wanted to put a lot of energy behind Android's device encryption, my suspicion is that they would be much farther along than they are now.