The 555 output will go high when a rising or falling edge is detected and will stay while the capacitor is charging.
And yes, the 555 is there to slow down the pulse and ignore if there's more than one in a short time, you can use the output directly if you want it faster
The circuit uses transistor to form a XOR logic port, the reason for this is to be easier to create this circuit in real life, if you like transistors of course...
But the choice is up to you in the end... using a whole IC for this seems to be a little overkill to me and maybe slower...?
