I suppose Naomi's referring to (among other things) the "Chariots ... made of iron" passage when she talks about the God of the Bible being not omnipotent. On the other hand, like everything else in the Bible, this particular passage is subject to interpretation, so maybe there are other passages that provide the same sort of thing? Anyway, see the discussion below for the relevant verse and a few interpretations of it:
https://www.quora.com/Why-was-the-Biblical-god-YHWH-unable-to-fight-against-iron-chariots
It doesn't particularly matter one way or the other, as you can still discuss the paradox. Myself, I think it only betrays a weakness, or vagueness, of language rather than the impossibility of omnipotence -- or, at any rate, sets a bound on God's power only in a pedantic sense rather than in a way that actually matters. Put another way, I think it shows that we haven't defined omnipotence in a way that's logically consistent, but that doesn't in itself mean that the concept of a supremely powerful being capable of creating (and destroying) an entire universe, and working within it to achieve whatever he so wishes (that is also possible) without any other impediment is inconsistent.
In that sense I see it as more of a language paradox, along the lines of the Cretan barber paradox, ie "if everyone on Crete either shaves themselves or is shaved by the barber, to which group does the barber belong?", or trying to resolve the two statements: "The next thing I say is true: The last thing I said was false."
There are other reasons to doubt the existence of God, of course, but this one seems to me to be about language instead.