1. Crucial would love to sell you the more expensive RAM if hey could. They could be lazy and not check things out, however.
2. The RAM modules do not connect to the processor they connect to the motherboard. If the motherboard wiring and/or circuitry did not implement the additional memory high-order "address line" needed to deal with 16 GB, you cannot access the upper 8 GB on the module. Besides, planned obsolescence is always a design requirement! :)
Plugged into a properly designed motherboard the i5 would indeed handle 16 GB.
But, if you can find someone who will take back the RAM modules if the don't work for you, you could go through the exercise.