In theory, that variable could already exist, which in turn would give a wrong representation of the value of %tongue.
If %tongue had a non-numerical value, then the inc would do nothing, and if it had, it would increase, whilst we'd want it to be 0 when the script is first started.
Yes, the odds are slim, however, I thought it was justified to tell him to type /set %tongue 0 first for that reason.
Greets