Question about Aukey Sprint Ultra PB-XD26

Yes (via the USB C PowerDelivery 3.0 output into the USB C input of the Surface).
It's a bit slower than the regular power adapter and my Surface Book 2 (13") also gets a "slow charging cable" warning, but it's still relatively fast up to 80%.

This powerbank itself is a charging artist! With the right power supply unit (56W!) it is recharged in no time! And yes, I also use the powerbank for a Surface (Go).
Here is the power supply:
Aukey PA-Y10 (3A, 56W, Power Delivery 3.0)

You need a special PD3 USB C cable with a chip like this one...
Xtorm USB-C Power Delivery Cable (C - C, 1m, 3.1)

Normal USB C cables are not designed for this because they cannot transmit very much power.