True. However, depending on the random digits involved (just the last four numbers, or were prefixes involved in the randomness?), you might be able to randomly call people in an area with a demographic most likely to achieve the desired result. Harlem vs. Manhattan, for instance...
So much depends on the finer points of the technique, and we don't have those to analyze.
If you call Manhattan only numbers ~ if such a thing is actually possible anymore ~ you have 1 in 10 who are Republicans. A poll taken there would look quite reasonable with 9 Dems for each 1 Rep. If there were a 50/50 split, that would be unreasonable.