It isn't. Doppler is used for nearby objects in relative motion; cosmological is used for redshifts generated by the expansion of space. People who know what they're doing never confuse the two. But sometimes pop-sci sources loosely refer to the cosmological redshift as "Doppler", and treat it as if it can be converted directly to a relative velocity, rather than a change in scale factor. (The two work out approximately the same for cosmologically "nearby" objects, but diverge significantly at large redshifts.)

Grant Hutchison