How do canned soup companies keep their noodles from absorbing all the liquid in the can?
Canning
I've been looking up canning, as I suspected it had much to do with the process of noodles not absorbing all the water. I've found this tangentially related post and quoting:

You CAN can pasta yourself. It is not difficult but, like the commercial caners [sic] you will need to make sure it is high acid (they add flavorless citric acid) but using a red tomato sauce works just as well.

I have also found this very interesting set of ideas:

The idea is simple and genius: combine par-cooked noodles, a bit of vegetable base, some raw sliced veggies, and a few seasonings inside a jar.

Partial cooking
On an article about partial cooking which has some good ideas on par-cooking. I suspect this, combined with the above factors, as well as vacuum-sealed canning, is what allows for noodles, and vegetables/potatoes, to not absorb all the liquid.
Dig deeper
If you wish to dig deeper, I have found a scientific paper by American Association of Cereal Chemists specifically on the topic of liquid absorption by noodles:
Noodles. V. Determination of Optimum Water Absorption of Flour to Prepare Oriental Noodles