English grammar

You did not quote the main clause so we can not parse this. I suspect that the main clause was presented in the preceding paragraph. An author might do anything if he thinks it will make his presentation more memorable.

Thanks! The preceding paragraph doesn't contain its main clause. The article:
"--- If he thought more deeply than the other great biologists of his age, which he did, it was surely because he read further, too.

Unsurprising, then, that he saw the life he studied as a thing
that could be read—and that could read itself. In 1960, during a
long conversation with two of the other giants of molecular biology,
Francis Crick and François Jacob, he was the first person in
the world—by about a second—to understand how cells read genes
to make proteins. In the 1970s he suggested a way of turning all the
genes needed to make up a human into a “Book of Man”—a precursor
to the Human Genome Project that he would go on to champion.
The most sustained project of his life was an attempt to learn
how a tiny worm read itself into existence from nothing but an egg
and a genome." from The Economist

Why doesn't the author use "That is not to say ---"? At least, it is grammatically correct.