FWIW Bank of Montreal is not a US stock, and it’s taxed at source in Canada where WH tax rates are 25%
I have never checked if there is a double taxing agreement between CA & UK.
IF there is an agreement and you get “double taxed” you can claim the WH tax from the tax authority in source country (In this case Canada) since you paid no tax to HMRC in UK inside your ISA there is nothing to claim back in UK.
HMRC does not pay tax to IRS. IRS taxes you at source. That page you linked half-arses this information. For US shares, if you have filled/submitted a W8BEN you already get taxed at a lower rate of 15% instead of the 30%. IF you have not filled W8BEN and you got taxed the full 30% then you can get your 15% back (out of the 30% you paid)
but like I said first, none of this applies to you because Bank of Montreal is listed in Canada.