AIUI the AFF W8 form differs from the one from the IRS. On
the IRS form the "foreign tax ID, if any" is optional, perhaps because on the IRS W8 Form has a box for "Individual", whereas the AFF W8 says something like Sole Trader.
Even their own FAQ says:
"Based on our understanding, due to the nature of the services provided by the affiliate program
we cannot accept a W-8BEN form from an Individual."
[...]
"as stated in the instructions to the Form W-8BEN,
you must complete the Form 8233 rather than the Form W-8BEN if you believe that you are entitled to an exemption from U.S. income tax withholding on payments by us."
[...]
"If you do not have US Activities, please click on the Foreign affiliate with no US Activities link and agree to the statement on that page. In this case, no tax forms are required."
Clear as mud.