But we have no way of knowing what undocumented wake up keywords are built in, or whether there are any other circumstances in which they will start to record, send and process audio.
Bullshit. Code review, disassembly, fuzzing inputs, monitoring network traffic. Talk to any security researcher before you spout off on what we have "no way of knowing".
There have been various rumours of Google, Amazon and Smart TVs listening in for shopping-related terms in order to target advertising.
There have been various rumours of Elvis sightings and lizard overlords. Cite credible sources or don't repeat shit.
And if they aren't doing that today, they certainly will be just as soon as they can get good enough local processing (which won't be hard in mains-powered devices).
This is the one thing you've said so far that I agree with. Amazon, Google, et al are motivated by selling stuff or advertising. Anything they can do to increase profit from that is likely to happen. The only reason I don't think it's happening right now is that the local processing requirements are higher than what we find in the relatively lightweight devices available today. The only ones that might be able to approach this computing-wise are smartphones, and they're too motivated by keeping battery life within reason to go very far with this.
The article raises the question: if they are going to do that for their own commercial ends why wouldn't we require them to also do similar things for social good reasons? Good question.
Remember what I just said about local processing requirements? Okay, now scale that up exponentially. We can't be just talking keyword recognition here, because voice recognition, as good as it is, still has a lot of trouble, especially with similar words like "grape" and "rape". It'll need full contextual recognition, which even the full-bore cloud "AI" systems haven't been able to even start to get right. Otherwise your phone or in-home device will be asking "are you alright" so often that you'll likely smash it just to get some peace.