In M.L. West's commentary on the Theogony, he remarks in his introduction to the Typhoeus passage (820-80) that it is 'one of the sections of the Theogony whose authenticity has most often been disputed', and then briefly summarizes and responds to six arguments that it is not by Hesiod. Here is the third, followed by an important critical principle:
Gaia's part in producing an enemy to Zeus' regime is at variance with her benevolence toward Zeus in the rest of the Theogony. Again, comparison of an Oriental parallel (Enuma Elis) helps to explain the anomaly: see p. 24. The assumption of multiple authorship is the most naive of all ways of accounting for contradictions in mythology.