Did I read in an answer somewhere here on Quora that spoken and written literary French are in fact two different languages — so much they differ?

Yes you have:

Andrew McKenzie’s answer to Why are written and spoken French so different?

To be fair, Andrew wasn’t making the claim explicitly, he says that some linguists would. He then lays out the arguments why you would.  And having a different underlying syntax is not a register difference, similar to English or other languages I know, contra what Riley Ertel says. It is a huge structural difference. Different language family-level structural difference.