Could someone elaborate on the underlined section please ?
What do we mean by 'proceeding as a main clause from the previous sentence' in this context.
The that-clause can be seen as a dependent clause preceding from the main clause, "They echo an in important principle...":
They echo an important principle; in that we shouldn't fear or challenge death, or the nature of death ; that we meet it half-way be valuing life and respecting and not harming others.
and so it should not be separated. Usually, a dependent clause can be the head of a sentence if it does not precede as a main clause from the previous sentence. However, as you can see if it were not separated it would be an extremely complex sentence.
I'd advise that you read this discussion.
https://english.stackexchange.com/questions/452627/correct-usage-that-they-as-complete-sentences
If you have questions regarding it, please refer to the source when you ask.
And later,
You have quoted 'proceeding', as if it were something that appeared in the text you are trying to interpret, and yet the text contains a different verb (precede) and a different verb form (without -ing), so your "quote" is not a true quote at all.
Please correct your question where necessary so that someone can try to answer it.
CJ