I'm boggled by this. I spent a good 4 months working on toffee--much of that trial and tribulation can be found here. In the past year I've probably made a few tons of toffee and most of that was by hand. Now we have a firekettle (which is a new learning curve & beast but that's another tale..)
I like to think of toffee like making a roux. You can't have excess butter or it won't bind to the sugar. So I'm really curious what kind of product they are making to have greater butter than average. I mean it's a butter + sugar product, plain and simple. Your ratio will dictate a few things but toffee is toffee is toffee until you start adding alternate sugars (like honey) or end of product additions.
Anyhow my toffee has about a 6 month shelf life before the elements start to seep in and the quality is not what I like. Luckily no where we stock lasts that long due to consumer habits but that's the longevity we give it before we would pull it.
Like it was mentioned toffee is hydroscopic by nature. It wants to absorb water, it LOVES to absorb moisture. In the summers here in the Carolinas where the average humidity is 90% we have to change the entire method of making it, and then speedily get it enrobed so that we can prevent the air from getting to it. If we left a sheetpan of toffee out over night it would have a liquid layer by the morning. Crazy. Even then packaging has some microlevel of porosity and that will eventually be the downfall of the product.
I like to think that at the heat level its cooked the proteins are broken down and the fats are converted to something more like ghee so you've got really no chance of rancidity then there's enough sugar that it's a shelf stable product since your available water is nill unless the environment adds it back. So I'd love to know how Enstrom's has done something so unique that it has this required refrigeration. It just doesn't make sense to me.
For us it's a fire and forget process. I wouldn't worry much about it. Run your own shelf tests and you should find it's something you can reliably not worry about.