Can battery health be increased back to 100

Probably, the link on top of this page which you had visited, before posting this question here, is shown under the "Related Article” that had said in first paragraph...

"All rechargeable batteries are consumables and have a limited lifespan — eventually their capacity and performance decline such that they need to be replaced. As batteries age, it can contribute to changes in iPhone performance. "

Probably you missed it.

The link on top of this page which you had visited, before posting this question here, is shown under the "Related Article” that may have given you an answer. 

You need to mention which part from the above article was not clear to you.