But that's what he said: "according to his [the rider's] clock" it would take about 3 seconds to reach the center of the Milky Way. So yes, he is talking about how fast the riders perceive it, which is all they care about, since they wrote off their entire families for dead before they left.

Now the strange thing about traveling at 1516c (i.e. far greater than the speed of light), is that the time dilation equation yields an imaginary time, in this example 1 / sqrt(1 - 1516^2). I'm not sure how that translates into a perceived travel time of 21 years.