How can a non-sine waveform can equate to combination of DC voltage?
You are misinterpreting the statement. Note that is says "a combination of DC voltage, sine waves, and/or cosine waves". Some waveforms, when decomposed into a series of sine waves, still have a DC offset. Note that sine waves and cosine waves have an average value of 0. Thus if a given waveform has a DC offset, than that DC must be combined with the sine and cosine waves to create the original waveform.  As a simple example, suppose the waveform in question is a single sine wave with a peak value of 1 volt but with a DC offset of 10 volts. Then, it can be decomposed as 2 components: a 1 volt peak sine wave and a 10 volt DC level. Again, the author is not claiming that all waveforms can be broken into DC, but only that some waveforms require a DC term in their decomposition to account for any DC offset in the original waveform.