I don't know the specific amount of data (as a percentage or bytes) accessed, but I think there are two main reasons you might want to know:

If you're wondering whether it affected the privacy of data created by people who use Facebook, the referenced article has a statement that it was not, but it appears this was added after the article was published, so you may have missed it.

If you're wondering whether it might be a small amount that a security researcher might collect to verify their report, from what I understand it was more than that.