If something is truly measurable might not be the right question to ask here. I am not an expert myself on that field either, but getting a significant result depends on more things than just a design change.
Anyway, back to the font color part. This also heavily depends on your the rest of your design, your target group and target devices. The article describes some of the needed contrasts to create good readability. There won't be a 'one size fits all' solution for this, you will need to rethink this for every design.
I've just read the article but the writer doesn't say the grey he calls 'Raven' is better than a grey with equal values. It's just the shade of grey he thinks will work best for his case.
The only visible difference between the two is that 'Raven' is slightly cooler than the 63/63/63 grey you describe. He probably chose this because a cooler toned grey matches the rest of the colors better than a warmer toned grey.
Truly knowing what will work best can only be achieved through testing and viewing results.