Yeah, it was a reasonable/effective solution before low-impedance cans became more of the norm (presumably with the rise of the walkman/discman, where for similarly cost-sensitive reasons you'd rather need higher currents than higher voltages).

While it may flatter the Sennheisers, a few hundred ohms of output impedance will have much less agreeable effects for a lot of cans/IEMs, so I just want to assure OP that his headphone dongle is behaving as a modern headphone amp ought to.