Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you with your wee one today.
You are correct that kidney damage cannot be cured. That said, we can manage kidney cats and support them such that they do enjoy a good quality of life even after diagnosis.
Now you noted this cat was vomiting and lethargic, if the vet thinks this is secondary to severe kidney disease (and nothing else), then they may need to treat this cat with IV fluids and injectable anti-nausea treatment. This will flush out the toxins and lower those kidney values that were tested. If they can get the levels low enough, this kitty will start to feel better and medical management (ie Semintra, Fortekor, phosphate binders) +/- a low protein diet would be indicated to keep her stable.
Overall, it isn't a case of them never recovering. Instead, kidney disease is a chronic progressive organ failure that will worsen with time. But if a cat responds to treatment, they can live for months to years with this disease. Of course, if she has very high kidney levels, is sick with this, and they don't treat; then she likely only has days to weeks left.
I hope this information is helpful.
If you need any additional information, do not hesitate to ask!
All the best,