I doubt there are any chemicals involved. It's been a while ... a long
... since I talked lumber grades with anyone, and then I dealt only with Oregon mills. I'm not sure how they grade lumber in Canada, but it's probably not too different.
I think the "Premium" simply refers to the grade of the lumber, which would involve the straightness of the grain, moisture content, color, and various other properties. The wood, of course, is simply from fir trees, probably Doug firs, but possibly the genetically modified "super firs". Either way, it's just fir.
IMO, premium quality lumber is probably more expensive than you need for building raised garden beds. Top-quality lumber is usually used in situations where appearance is important, not so much for more utilitarian purposes. The finest grades won't be any slower to rot than a lower grade. But, as I say, that's just my opinion. I would probably opt for the next lower grade of lumber and put the money I saved into filling the beds with extra-nice soil.