The short explanation
Send 1080p to a 4K panel and something has to invent the missing pixels. Send 4K to a 1080p projector and something has to discard them. That is scaling, and every display has a scaler built in whether or not you think about it.
Dedicated external scalers exist because the built-in one is not always the best available, particularly for film sources on a projector.
Where you run into it
The rule is: scale once, in the best device. Letting a player scale to 4K and then a projector scale back down to its native resolution runs the picture through two conversions, and it looks it.
The advertised capabilities decide where the work happens. A source that thinks the display is 1080p will scale to 1080p at the source. Change what it is told and you change which device does the work — without touching a single scaler setting.
How the DR HDMI 8K handles it
The DR HDMI 8K is not a scaler. It does not change resolution, and it passes the signal through unchanged. What it does control is the resolution the source believes it should output, which is what determines where scaling happens in your chain. If you want the projector's scaler to do the work rather than the player's, that is an EDID decision.
Want to test your own HDMI chain?
The DR HDMI 8K is sold through the HDFury.eu web shop.