Think of directions to someone's house. Left at the petrol station, right after the church, third gate along. What nobody writes down is the address — the place you are actually trying to get to. Follow every step exactly and you can still end up lost, because the petrol station closed two years ago, and the directions have no way of telling you that.
Everything we build is like those directions. It stores the steps. Where they were meant to lead stays in someone's head, or in a note nobody updated, and eventually that person leaves and it is gone for good.
A meaning substrate is the missing half. It is a store for what a thing is for — written down properly, in a form a computer can read and check, instead of a form only a human can guess at.
Once the meaning is written down, it becomes the thing you keep, and the steps become something a computer can work out from it — the way a map works out a route from an address, and quietly finds another one when a road closes. Over and over, differently each time if it needs to. A different machine, a faster way of doing it, a change in the world underneath: same meaning, new steps, and you didn't have to rewrite anything.
It also lets you ask questions nobody can really answer today. What is this thing for? Does it still do what we said it would? Did that change alter what it means, or just how it was worded? Right now every one of those questions is answered by a person reading through the steps and guessing.
Nothing about this is really about writing software. Anything that carries meaning has the same problem: we keep the thing itself, and lose what it was for.
Each of those fields has spent years building its own way of working out meaning, and none of them can talk to any of the others. Whether one store could hold all of it is an open question — nobody has done it, and it might not be possible. That is the half worth arguing about, and it is worth reading not because it is right, but because it might be.