Hi, I’ve just got into playing around with these things myself and think I’ve run into the same issue, apparently a lot of power banks have a power threshold to stay on and if a USB device is using less than that then it will stop supplying power and turn off to conserve battery. It makes sense since they are mainly used for topping up phones which will draw 1 or more amps while charging then drop considerably once they reach full charge at which point you probably don’t want the power bank to continually discharge. But sadly it makes them useless for low power devices like the Xiao BLE.
However you say that it is not turning on at all which makes me think you may also have the same issue as in the thread you posted where the line of code “while (!Serial)” will loop indefinitely and never start running the blink code, I got around that by replacing it with this code:
int serialAttempts = 10;
for (int attempt = 0; attempt < serialAttempts && !Serial; ++attempt)
{
delay(50);
}
That way if it is connected to computer you get the serial working, and if not then it will stop trying after half a second and run the rest of the program. Unfortunately my program still only runs for about 10 seconds before the power bank shuts off 
.