Its just the AJ Bell site says
You donāt need to complete either form if you hold your investments in an AJ Bell Youinvest SIPP. This is because when in a SIPP, US investments are automatically exempt from withholding tax
And as mentioned above the Hargreeves site says
WHAT IS A W-8BEN?
A W-8BEN form means we can claim a US tax reduction for you on your dividends and interest from US shares. Withholding tax rates can be reduced from 30% to 15%, or to 0% if your shares are held in a SIPP.
Are you sure as it is also mentioned in here
Buying US stocks using a UK ISA or SIPP (whatinvestment.co.uk)
The cost of doing so is usually higher than when buying UK-listed stocks. As well as standard dealing charges and management fees that can vary by provider, there are also usually foreign exchange fees. They are charged for exchanging sterling into dollars when purchasing US-listed stocks, and vice versa when selling them.
Although some sharedealing providers offer US dollar-denominated accounts that could reduce foreign exchange fees for investors who are likely to trade US-listed shares regularly, ISA balances can only be held in sterling. Therefore, US-dollar denominated accounts may lack tax advantages.
In order to reduce the amount of tax paid on dividends received from US-listed shares, a W-8BEN (US tax) form should be completed and submitted to a share dealing provider prior to purchases being made. This reduces US-levied withholding tax from 30% to 15% (or even to 0% in the case of a SIPP). The form is fairly short and worth completing for the tax benefits that it may offer.
Also found this
These days itās easy for stock market investors to buy shares in overseas companies, amongst the most popular being US companies.
Foreign dividends are often subject to withholding tax - the overseas company will deduct tax before paying you the dividend. However, the UK has double tax treaties with many countries that reduce the amount of foreign tax payable (usually to 10% or 15%). In the US the dividend withholding tax rate is normally 30%.
However, in terms of the double tax agreement between the US and UK, the amount of withholding tax can be reduced to 15% by completing form W-8BEN, issued by the US Internal Revenue Service (IRS). Most online stockbrokers will handle these forms on your behalf so the process is relatively simple.
The double tax agreement also provides a specific exemption for pension schemes, which means US dividends can be received tax free if the shares are held inside your SIPP or another pension scheme. The double tax agreement does not, however, recognise ISAs. ISA investors are still subject to withholding tax.
If your overseas shares are held outside an ISA or SIPP you may have to pay UK income tax on your overseas dividends if you have used up all your 0% dividend rate tax band. However, you may be able to claim Foreign Tax Credit Relief when you submit your tax return. This allows the overseas tax paid to be deducted from the amount of UK tax owing. Bear in mind though that the amount deducted cannot exceed the UK tax payable on that income.