Affinity have got themselves in muddle here. DPI is an instruction to the printer defining the density of ink dots to be used in printing process.
The image on your screen is unaffected by DPI.
The screen image size and the ultimate printed image size are set by the number of pixels per inch, PPI.
PPI is the only dimension that matters - until you are printing. If you set the DPI below 4x the PPI the printer will reduce the amount of ink it uses and the image will print with less contrast and reduced clarity.
Print manufactures always quote DPI because it looks good on the box; Maximum 2,400 DPI looks much more tempting than the meaningful resolution maximum of 600 PPI. It's value is in the fact that the higher the dpi the higher the quality of the final printed image.
So ignore DPI unless you are in the process of printing. In any event most domestic printers generally default to 300ppi and 1,200dpi. Espom go their own way with 360ppi and 1,440 dpi