I have a hard time believing it also.
However, Washington’s army at Yorktown including militia was 11,000. That would mean an upper limit of that German’s claim to be about 3000. Number of free blacks 1790 census in the USA, roughly 56,000. I don’t see it as impossible to have maybe 2000 in Washington’s army. (Note the regulars were 8000.)That might be enough to make a visual impression on that German officer. I am sure he didn’t count them. There were a lot of free blacks who were able seamen in the merchant ships, privateers, etc.
Again I would like to find the source to that claim.
But not regulars. There might be a high number in militias. However, our “Politically Incorrect Guide to the American Revolution,” out this fall, says that the war was disproportionately fought by Scots-Irish.