The only reason I can think of them saying that is, if you keep your superthrive/water mix in an uncovered container it could possibly lead to algae growth in the water (since algae spores are ever present in the air around us) being a plant growth hormone it doesn't care if it's feeding green algae or a higher form of plant life. But if you mix up your superthrive water in a gallon jug with a cap then don't worry that's what I do with no ill effects. I usually use 1 gallon of fertilizer water per month that's two sessions (every two weeks) so mine sits around for 2 weeks ontop the fridge (keeps the water warm-which many plants appreciate).
Just to give you an idea what fertilizer recipe I use:
Add the following ingredients to 1 gallon waste aquarium water (you can substitute distilled or RO if you don't have an aquarium which needs water changes - I only use aquarium water for fertilizing then it's strictly R/O the rest of the time)
-5 drops of liquid Aquarium trace minerals
-1/8th of a tea spoon of Orchid Fertilizer if you can find "Jacks" or another brand which has balanced numbers like 20-20-20 and trace minerals you can leave the aquarium drops out but plants do need Iron and those other elements to give you their best colors. Red tinted species especially require lots of Iron to show their "true colors".
-Occasionally a drop of Superthrive, possibly evey other month (or when I think of it)
The key to doing any kind of chemical fertilizing with Nepenthes is to err on the side of using too little rather than too much. I learned that the same goes for feeding the plant bugs as well-at least, in old pitchers!
Now my alata is uneven (looking great but uneven)!