This what I was thinking. The device has voltage and current limits built in. Chargers have various modes of voltages and current outputs, and will not just pour any amount, the device has rules to allow only certain amount in. So, that statement on the manual made no sense. Probably not written by an engineer.
Is there an engineer from Qudelix give us a precise answer?
I am not from QUDELIX, but if you agree, I can explain something.
The charger is inside the device, always.
What we call 'USB chargers', in reality it would be more correct to call them USB power supplies.
If they are compatible with the PD or QC standards, they are able to supply a voltage greater than the standard 5V. But this supply of greater voltage occurs exclusively upon request of the device being charged, after specific negotiation.
In the case of the 5K, it does not occur, and therefore any USB power supply supplies the standard 5V.
Regarding the 80% charge limit, it actually has value and protects the internal cell, especially if it is left for many hours in a fully charged state.
In other words, the cell left charged at 100% rather than 80%, undergoes greater degradation.
But these are known recommendations, which are valid for practically every device equipped with lithium cells.
Less subject to this type of ‘degradation’ are lithium iron phosphate (LiFePO4) cells, but they are rarely used as single cells in electronic devices, for various reasons, one of which is their lower nominal voltage (3.3V instead of 3.7-3.8V).