If that is the original intent, why did that not get added into the law? Explicitly specififying that the exception only applies to emergency backup situations would have solved this, right?

"Emergency" is a spectrum.

Would you be OK with one specific generation station temporarily emitting 3x more than legal limit (e.g., from the hours of 2pm to 6pm) if the alternative is a 5% increase in absolute probability of a multi-day regional blackout? What about a 10% increase? Where is your red line?

My reasonable guess is that they did not want to make those case that need off-grid generation too hard to build. And never considered that anyone would build any actually significant amount of off-grid capacity. At certain threshold grid just makes economic sense. So rational actors would never do that.

Sadly AI companies are entirely irrational actors so this really was rather hard to foresee...

Because congress passed the law in 1970 and did not expect we'd lose the ability to pass legislation that fixes minor problems with the original laws.

Since congress can't do that anymore, to maintain a functional civilization we have to turn minor loopholes in the original law into load-bearing aspects of an industrial society.

Clearly codifying intent in a string of bytes is hard, especially if an alternative makes a nice sound folding into a pocket.

The law is not a set of hard coded rules. It is an agreement between humans, tied up in unspoken conventions. The principle one being, don't be a dick. The law always breaks down when it is treated as a hard and fast set of instructions that can be "hacked" by slavish adherence to precise language.