[hsalaw]

The 400 years he speaks of are counted starting from well before the Revolutionary War.

I'll say it's "well before" the Revolutionary War. 400 years back is 1608, which is even well before the first Pilgrims arrived in the Mayflower, which was in 1620.

[Windflier]

Google “400 years of slavery”. You’ll see why people use the phrase.