Output impedance for microcontroller minimum 5kOhm
The datasheet refers to the minimum impedance you can use at the output of the DAC. (Minimum Resistive Load with buffer On, page 163).

Which means, if the chip is powered at 3.3V and 5kOhm it can drive 0.66mA max.

This won't be enough for your earbuds, so you will need an external buffer.