I would suggest all hutches are treated on arrival, from a long discussion I have had with the people at Ryedale hutches they treat their hutches before they go out, however they do recommend that all hutches they sell are treated within 3 months of purchase. I would assume that this is the same for the Happy Hutch, however if it isnt you will not have damaged your hutch but probably done it a favour.
I would strongly suggest that you do not use creosote as the long-term exposure to low levels of creosote, has resulted in skin cancer in animals as well as humans. In 2003 the European Union banned creosote for amateur and unlicensed professional use, due to concerns over these health effects, recent research had shown that the risk of skin cancer had perhaps been underestimated previously.
There are many animal friendly wood preservatives available including most of the DIY own brands and also ones like cuprinol. If you wish to make your hutch more waterproof you can add a small amount of cooking oil to it, it will take longer to dry, however you will notice that the water will literally run off it instead of some soaking into the wood.
If you want your hutches to last I would suggest doing them on arrival and then once a year after this.