The videos are already public on YouTube.
Maybe they just wanted to send this video to a friend and didn't have the technical understanding that this will then be visible for everyone searching for it on YouTube.
1. This isn't really privacy breaching. For someone who taps the "share to youtube" button without knowing what it means, sure, but even that is pretty explicit that you're sharing it. Not sure why the article itself says people didn't know what the button would do before tapping it, so I'd like some further explanation of this point.
2. It's opt in, not opt out. Spending time with most "normal" people has shown me that very few people give a crap about going into settings menus to configure exactly how ther data is used or collected, or otherwise switching to a service that gives them that control. When HN complains about privacy being dead, they are complaining about this apathy end how it gets exploited. This feature does not exploit that apathy.
3. This gives us something that we actually want. When most services invade your privacy, it's usually for things like advertising, targeting content, and data brokering. Things that I know I personally have a lot of issues with, and I feel I'm not alone. This button doesn't do those things, it just gives us interesting videos. So much so that most of the fascination with these videos is that you can feel the absence of those issues.