@reika,
That's just it -- I'm proposing that it
is revolutionary to them. To be
very clear: supply trains continuously running from one's home domain to the front lines
was not a thing in the Sengoku era -- at the very least, not when taking into consideration how military logistics work since the 20th century. Each domain the Oda (and others) conquered was conquered specifically for two reasons: expansion of their domain for the purpose of acquiring power and prestige, and establishing new supply bases near the front lines from which to launch invasions fully supplied. Thus, for Shizuku to come out and say, "why don't we set up a system of supply trains that can indefinitely resupply our front lines during campaign season from our home base, where it's safe?" would be groundbreaking for feudal Japanese commanders who thought of supply chain logistics as "we carry what we can and forage what we must." Think about it: rather than have an army's entire supply for the front line come from near the battlefields, where enemies could raid and forage and you might need to abandon at a moment's notice, you get your supplies from a safe location where farmers and craftsmen can mass produce in relative safety.
EDIT: I would also add that this also removes infrastructural concerns for attacking armies beyond the need to keep roadways open and well-maintained. If you don't have to worry about keeping a domain intact as you conquer it for fear of losing out supplies, that opens up
a lot of options for conquest and immunises you to a certain extent from scorched earth policies.
She's basically modernising warfare in a way no one in the Sengoku era could've come up with because that kind of leap of thought would've been beyond their current understanding of warfare. Preceding chapters already clearly showed that aside from Nobunaga (who quickly grasped the possibilities once Shizuku showed him her abilities), other samurai saw agriculture as merely "growing just enough food for campaigns and survival." They couldn't even grasp the notion of a technology campus until it was basically laid out to them. Nobunaga and other major samurai figures have even expressed a lack of knowledge about Sun Tzu's
Art of War, which today we would consider basic reading for military figures. From what we could see when she first practised basic arithmetic, their understanding of math is also vastly different from modern standards. It's not because they're dumb, though, but because their upbringing and dominant social norms, as well as lack of comprehensive modern education, forced them to grow with a skewed view about warfare and socioeconomic progress. Add to that geographic features, which you correctly point out, that would make thoughts about "nationwide supply trains" seem ludicrous, and that would lead their ancestors to dismiss the thought entirely, making their current understanding of logistics dogmatic.
Thus, it's easy for us to say, "oh, that's obvious. Clearly, they would know," but in reality,
no, they wouldn't. They're too constrained by traditional dogma about logistics to even make that leap. That's what made Nobunaga so remarkable (and such a good overlord for Shizuku): he hates abiding by tradition for tradition's sake and, in this manga, is shown to be quick in grasping and applying the concepts she teaches him -- even when they're expressly not of military nature (for example, the whole stress thing). Thus, even though he could've just had Shizuku tell him, and only him, about logistics and other matters, he often seems to be using her as a vehicle through which to shatter traditional preconceptions about warfare and socioeconomic progress among his commanders.
At least, that's what I believe is happening. I might be wrong in subsequent chapters.