Let's clear up a few points.
Firstly the outputs of the 4017 are totem pole so that each of the outputs are kept low via a transistor when they are LOW.
This means the voltage developed across the current-limit resistor (390R to 470R) will be approx 9v when the chip is on a 12v supply. This 9v will appear on the cathode lead while the anode will be very close to 0v.
This reverse voltage may be too high for some LEDs.
Secondly "100mA/h and 100mAhr" are two different things.
If you have ever done any capacity calculations you will understand the difference.
100mA/h is a flow-rate equivalent to 100 milliamps flowing for one hour. I say equivalent because it may be 200 milliamps flowing for 30 minutes and nil for the next 30 minutes and this continues ad infinitum.
100mAhr is a "capacity" - generally referred to the capacity of a battery. This energy can be drawn off at 200milliamps for 30 minutes or 10 milliamps for 10 hours. Once the 200milliamps for 30 minutes is extracted, the energy ceases as this is the total energy available for the cell or battery. The 100mAhr can also theoretically be extracted at 1amp for 12 minutes or 10 amps for 1.2 minutes - so long as the two values total 100mAhr when multiplied together.
How does that circuit achieve a five seconds delay after a complete cycle before restarting?
The purpose of the 100u is to generate the "time delay." The 100u is pulled HIGH via the 10th output and this turns on the transistor to inhibit the 555.
The 100u gradually charges via the 10k and the 100k monitors the voltage across this resistor. Initially the voltage across this resistor is large and as the 100u charges, the voltage drops. Eventually (after 5 seconds) the voltage is less than 0.6v and the transistor turns off and the 555 produces 10 cycles before being inhibited again.
The 10th LED is not fitted so the display turns off completely.