I googled for that reddit thread (Gawd, reddit!), and the paragraph before the line you're citing reads like so:
That's the "asymmetry" part of Benatar's argument. He argues that the
nonexistent cannot be deprived of pleasure (they are already at peak
deprivation), and that the nonexistent will not experience pain (they
are at peak lack of pain); thus, they are at a peak good state. But
once you ask if the nonexistent can be deprived of pain (which
according to the "cannot be deprived of pleasure" assumption, they
cannot), the asymmetry falls apart.
Basically, the fallacy is this asymmetry: On one hand, it asserts:
The nonexistent cannot be deprived of pleasure and will not experience
But at the same moment, it ignores the parallel assertion:
The nonexistent cannot be deprived of pain and will not experience
The first assertion makes nonexistence sound like a positive; the second assertion makes nonexistence sound like pure hell.
The real problem with this argument is that it attributes a quality - namely, the capacity for experience — to a negational class. That is always problematic. Consider the difference between saying "men have hairy beards" and saying "not-women have hairy beards". The first is arguably true, at least in principle. The second is ridiculous, since 'not-women' includes chipmunks, goats, trees, glaciers, and planetary nebulae, none of which (aside from goats) are notable for having hairy beards. Nonexistence is characterized by the absence of experience, and only by becoming existent can we experience the qualia that we determine as pleasurable and painful.
The corrected assertion would be:
The nonexistent cannot be deprived of anything and will not experience
Which is neither intrinsically good nor intrinsically bad, except as one adopts an optimistic or pessimistic attitude.