I would avoid these at all costs to be honest.
If you can't get a fact sheet on what chemicals are specifically used to treat the wood (the store might be able to provide this information) I would assume that at least some of the chemicals used as preservatives may be harmful to guinea pigs.
Utility treated means the wood has had various treatments to stop it rotting or being damaged by natural elements over time, and because this wood is designed to be used outside, the restrictions on what it can be treated with are less stringent that wood designed to be used inside or in the construction of items that will come into prolonged, daily contact with people.
Basically they fling a lot of heavy duty chemicals at it, but in fairly low concentrations, so they won't harm you if you use it to make a deck and sit on the wood.
But your piggies might want to chew it and this could be a problem.
Go for an untreated wood and then treat it yourself using an animal friendly wood stain.
