Well of course they assumed he was a human being - what else would they assume it could be?
A demi-god, a deity, or any number of entities we now find strewn across many an "Encyclopedia of Greco-Roman mythology" and the like. That was the norm. The only Greco-Roman gods for whom we have writings by authors who place these figures into a geographic, social, temporal, and cultural context are figures such as Alexander the Great, Augustus Caesar, & Jesus, and a few others.
That is not evidence for historicity.
Of course it is. The fact that even the ancient Greeks doubted whether Homer ever existed is pretty strong evidence, as it tells us that those who did not have the conceptions of history, biography, culture, myth, religion, etc., such that they had any historical-critical methods upon which to draw still doubted one of "the poets" is evidence of a similar type. That is, even after the dual person had all but died out it, was reserved for "the [two] poets", (meaning Homer & Hesiod) and the works attributed to them were received myth
par excellence. Had nobody doubted, this would tell us little by itself. Had we found authors spread out in space and time within the Hellenistic, Romanist, and even late classical eras who wrote polemics against the classical Greek reliance one (Homer) whom they criticized the Greeks for believing was an historical figure, that two would be evidence.
As it is, we have the people who most relied on Homeric epics challenging this Homer themselves, and no real interest in attacking the Greeks for their beliefs in general even after they were conquered.
For Jesus, were it possible for those like Celsus (or even Julian) to refer to traditions that indicated no historical Jesus existed rather than e.g., criticize him for being illegitimate, this wouldn't give us that much (certainly not as much as the utter lack of such critiques amongst the vast piles of anti-Christian polemicist writings that survive). That those such as Celsus affirm Jesus' historicity is more significant. That there is no plausible explanation for
only our Christian sources without positing that Jesus was an historical figure, let alone all our sources, would be enough; that the we actually have evidence from not only from a Jewish author Like Josephus but the reports/letters from Pliny, Seutonius, Tacitus and probably more gentile sources (were we to use the standards mythicists apply to non-Christian sources, we'd say "certaintly") is simply icing on the cake: mythicist rely on accepting mainstream historical methods accept when it comes to the historical Jesus, in which case they don't just abandon said methods they dismiss the entirety of the historiographical approach. They don't seek to understand our sources but to explain them away while offering nothing remotely plausible in return.
The unfriendly sources would be more likely to question the miraculous parts of the story than the humanity of its inspiration.
Wrong. Celsus wrote these off as nothing special. Anti-Christian polemicists (at least until Constantine started making Christianity a way into imperial good graces rather than e.g., execution) relied on logical critiques against both Christians and their founder which ranged from alternate traditions for his birth to the impiety of his acts.