Where is the DC going to come from ... unless you consider zero volts a DC level? When current is flowing in the triac it is AC although not a pretty sine wave.
Cheap/poor QC dimmers may be unintentionally providing some rectification to the output signal if the triacs are not providing equal gating for + and - current, just like if you clipped the waveform with a diode, only not as severe. So, instead of getting, say, equivilent to +/-60V, you get +65 & -55V: superimposed 60VAC+5VDC.
