>Why would the last group of N people not subsistence farm?

Why would they be able to reinvent that, when none of their ancestors for how many generations did so? Is that something a person can do well, do you think, with no prior experience or expertise? What if they get it wrong, they'll starve?

We could ask why they could reinvent all the technologies that people from prior eras of agriculture could manage? Will they instantly be able to make their own ropes, do you think? Have you ever made rope? Does it not count as technology if it's not a transistor etched into a silicon wafer? But previous eras of history did utilized that quite a bit for their agriculture. Are they supposed to make due without? There must be a hundred different things they won't know how to do, but were necessary for agriculture in any era of history you might name, but that you can't name because you know nothing about it.

Technologies, ones so mundane that you don't even recognize they exist, permeate the world. They're lost and then they're gone because a replacement was better. But when the replacement disappears, those lost technologies don't spring back into existence magically. Civilization is "path dependent", it doesn't get knocked back to previous tiers because those previous tiers cease to exist once we've moved on to the next. And it's really hilarious to me that not only are you ignorant of this, but you're snarky about it too.