Ok, now it's starting to make a little bit more sense to me.

They massively oversize the second stage and launch it almost empty, so it can just do the 5 km/s or whatever is needed for orbit. Then they launch multiple tankers to refuel it.

They don't need lots of engines on the second stage because it's launched light.

So it's pure chemical propulsion.

But still with only 4 km/s exhaust velocity, no matter how you slice it, the mass ratios required from a single stage get impossible, ie even if the rocket tanks and engines didn't have any mass, payload sets an upper limit to the mass ratio.

EDIT: So looks like it has 10 km/s delta vee. Not 30.