The carbs - fibre = true carbs is accurate in some countries, not in others. If the fibre is listed at the same level of indentation as the carbs it is already removed. If the fibre is listed indented under the carbs it is included in the total fibre number and the subtraction is needed.
For example
Carbohydrate
-
Fibre
Or
Carbohydrates
Fibre
Fibre being critical is not the most well supported thing. There are definitely ways it could be useful, mostly by having bacteria break down the fibre into ketones which are then absorbed by the gut. This is useful because some of the ketones are quite powerfully anti inflammatory. That said, some people have gut issues like Crohn's disease which make them unable to have fibre safely pass through their gut.
Biology is complicated, we are all stuck doing an experiment of one, but hopefully we can figure something out. I find it all fascinating but the most important thing to me is my partner currently can go whole days without a seizure. If I never understand how and why but I get to have that be the case then I am content.