Illegal Substitution??

[GPC2]

Team K in scrimmage kick (punt) formation. 12th man of K (K58) runs onto field thinking he's 11th man. He got to the middle of the field and was waived back, so he ran back off of the field. I was HL so I threw my flag for Illegal Substitution. My R waived it off and said there was nothing wrong with the play.

The reason I threw it is because from the rules, a substitute who comes on the field and becomes a player must participate in the down. However, after reading the rules book it doesn't appear that K58 ever became a player since he never a)lined up in the formation or b)gave an audible signal for someone to come out.

I guess my question is was this Illegal Substitution or not?? (I am not sure it was now)