QBO behaves the same in private and non-private modes of Safari on the iPad. This is true with the default display width, and with the browser configured at its default 100% zoom setting; the user never needs to zoom in or out to get the zoom message.

There is a workaround, which is to tap the hamburger icon at top left. This makes the sidebar menu at left disappear, and then the register and other screens display at full width without the need to scroll.

At this point I believe we're dealing with a display bug in the web-based version of Safari on the iPad. Correct behavior would be to autosize the column widths to show them in full at 100% zoom, with or without the sidebar present.