This is based on the assumption that controlled release fert prills continue to release fert into the soil as long as it is damp. The reality is the technology relies on osmosis so the nutrient solution will only seep out when there's lower concentration outside than inside the little balls which means that nutrients can only get to a certain concentration in the soil. Given the potting soil makers add the least they can to save costs there's almost no chance there will be toxic levels of nutrient in any soil mix no matter how long it has been stored.
Any excess nutrient will be in water soluble form so can easily be removed with a good flushing of water when you water plants into the new soil.
I've been using soil mixes, sometimes stored for several years, and never had any adverse reaction. The warning could just be to protect manufacturer from possible litigation but could also be encouragement to keep buying new product rather than making good use of stored product?