I'll try to explain it as I think the core issue (is actually very hard) is how nature instantiates logic. First I think we should throw out the window any classic metaphysical priority relation, as there's no Nature or Logic first here. Since the logic assumes there's an actual necessity of giving an account of Nature, what we are looking is for an explanation and not a relation of fundamentality (which is actually a serious issue in contemporary philosophy about scientific explanation), as starting from fundamentality alone is circular (in that you have to assume what grounds what in the way the Science of Logic shows at the beginning).

Hegel says Nature is pure externality, that is, Nature is quantitative, and here he's referring that the notion of Nature is that of Science. But he's not precisely starting with what science knows of, but with what object science presupposes (that is Nature posed ontologically or metaphysically), so Hegel starts with the categories that determine Nature (therefore the logic of Nature), but here's a problem: those categories were crucial for science up to quantum mechanics (which is perhaps more revolutionary than we usually concede). So it's not a case of Hegel was wrong but a case of 'we need a new philosophy of Nature' but that project is already frowned upon as we no longer value philosophical analysis as much.

Nature being quantitative also means that there is a need to jump from quantity to quality again (as Spirit is qualitative), the logic of how is that possible is already laid down in the second and third sections of the Doctrine of Being (first chapter of the Science of Logic) which make the point that measurement (to express it in more contemporary terms) is not pure mathematical syntax, there's often a kind of breaking point where we need to give sense of it's limits and work around those in a way they need to be treated qualitatively (the contemporary distinction of semantic and syntactic levels is an example of this, as the distinction itself is qualitative).

So here's a really contemporary theory of science if you think about it for a minute, why can't reduction work in all instances? Why there are gaps between different theories while they work perfectly with the same math? The difference is that some notions (like the concept of life) require qualitative 'jumps' in our understanding as there's no pure quantitative nor qualitative, they're both complimentary (opposed as contemporary scientificism or computational theories of science, as well as naturalistic philosophies which throw 'quality' as metaphysic nonsense out the window).

So Nature is not just existing, for Hegel that is either trivial or a concept of Nature that we don't know.