And the blog post points to a book written by the one historian with access to the Lee papers and slave ledgers.
In other words, you have no proof.
Rightscoop.com posted an article about Lee on the anniversary of his birthday. Your blogger was the first person to comment (probably does regular searches for mention of Lee, so he can jump in). The guy was raving like a loon.
If a nut like that was my "source", I'd keep it shut.