I haven't wrapped my head around it either. It seems super relevant, but I haven't worked out exactly how it fits in with today's technology. It hit home for me more as food for thought and reflection.
Thinking about it bit more after reading your comment...
I was thinking more in terms of Google, Facebook and Amazon than smartphones, but I could see that too. I feel like smartphones in and of themselves are tools, like hammers that can be used to break things or build things, but only become tools of enslavement when you load them up with apps and then it depends on the apps you load on them.
I see Google and Facebook as massive supercomputers with huge teams of engineers whose job is to keep you on their site and monetize you. So rather than a simple tool (1995 internet) that can free people (more information, more connection, more location independence, but to primitive to subject you to minute surveillance and sophisticated emotional manipulation), we have a complex tool that subjects users to intense surveillance and plays on our minds in ways most of us don't understand, not even the engineers who are building it.
They collect a huge amount of data and optimize for a handful of metrics that seem morally neutral (dwell time, engagement), but which function by playing on our worst impulses. We are seeing those impulses play out in ways that erode our democracy, erode the basic civility in our local communities, play on people's fears, encourage girls and young women to hate their bodies, wrap people in toxic virtual communities to alleviate their loneliness rather than promoting stronger physical communities or healthy virtual communities, all while pursuing apparently morally neutral metrics like dwell time or CTR.
So as these tools become complex, using massive data and algorithms that tease out of that data patterns that no human analyst could see, they become "tools of enslavement."
Something along those lines, but just felt rather than thought out as above, is why the quote hit home. I'm not sure the above is well-expressed and it certainly is incomplete and you could certainly marshal many counter-arguments, but some vague sense of the effects of the great power these data-driven, personalized advertising conglomerates (Google, Facebook) have is what made me say that quote hit home.