Food particles will decay properly in a septic tank. I am wondering if you are confusing or the person who told you is confusing what is said about a garbage disposal with a dishwasher.I was told it was because of food particles.
It is recommended that you should not use a garbage disposal if you have a septic tank.
http://timfrankseptic.com/are-garbage-disposals-ok-with-septic-systems/
http://paradisevalleyseptic.com/can-i-use-a-garbage-disposal-if-i-have-a-septic-system/