I can't speak for the parent post of course, but what I think they might mean is this: social democracy eventually devolves into neoliberalism (or worse), which, as you rightly point out, is geared towards making life much, much worse for the many to the benefit of a shrinking few.

To see some examples of this, look at the arguably social democratic Keynesian US from the fifties, and where they're at now. Similarly for the United Kingdom. Labour used to fight for social democratic policies, and since the advent of "New Labour" (Thatcher's greatest victory), they're essentially a right wing party. For a much more harrowing example, look at Allende in Chile and the US sponsored coup by Pinochet.

In general what you see is that the social democratic status of a state is an unstable one. Roughly speaking, the people who would benefit from neoliberal policies happen to have a lot of money with which they can and do influence politics. There's campaign funds, lobbying, but much more importantly owning the media[1]. Propaganda is incredibly powerful (we are not immune), and it has been used successfully time and again to get people to vote against their own interests, such as abolishing social democratic practices.

[1] In the case of Chile, and for example Indonesia, the mechanism is much more violent, but the principle the same. Capital is spent to successfully influence politics in favor of capital.