Good call, I missed that aspect of horizon. Though if they could've transitioned to a managed db / app service provider it might've provided a way to keep going. Makes me think the mini-pivot was "a dollar short, a day late", and that perhaps managed PaaS could provide good revenue models for open source infrastructure.
When Horizon was announced, that's when I stopped even considering RethinkDB for new projects (even if it was a good fit, feature-wise). Call it a "business smell", but I'm not surprised to hear they've wound the business down.