Except pretty much all printer drivers these days can down convert from letter to A4.

Margins on left/right might be skinnier, but length wise US letter fits.

Since they are talking about physical paper size, I assume they meant that an A4 printer that physically only takes A4 sized paper would not be able to print on letter paper stock, as its physically too big to fit. I can print any size onto a business card too with down converting and messing with margins.