What you won't understand because you are new to comics is that the Swamp Thing was a muck monster - it started off as a horror title. And Alan Moore took the title and sent it in a completely new direction without ever contradicting what had gone on before.
So many of todays writers just want to start over and say "that never happened" - but Alan Moore instead said "that is what happened - but the reason why may be different than you thought it was...."
Another book that you would enjoy is "DC Universe: The Stories of Alan Moore" - it is a selection of stand alone stories he did of different characters in the DC Universe. It is again a great read and nice to see his view on a variety of heroes.