Actually, 30 years ago most analogue sources had around 1 kOhm output impedance. At that times, opamp outputs were not so frequent end even in that case first generation opamps were not able to drive directly a cable capacitance or sustain a short circuit, so output resistances were ubiquitary for stability and protection. This was perfectly acceptable because, at those times, standard input impedance was 47 kOhm or even 100 KOhm.
What you probably do not know is that Denafrips DACs are R-2R ladder DACs, which is an ancient if not the oldest way to do digital to analogue conversion. Just a series of calibrated resistors directly connected to the digital bits. This is the simplest way, which does not involve complex signal elaboration as in delta sigma converters. For the same reason, the less possible signal manipulation , the output stage is a simple low pass passive filter, with no active components to lower the output impedance.
This is no "poor" design. On the contrary, is quite sophisticated to do properly, as you need precision in the resistor matching in the order of 0.005% to achieve 20 bit resolution.
)
