I can see no other reason to judge this rule as invalid. This rule just slides in timewise. This rule follows the current restrictions. All rules must include the explanation of the symbolic expression given in another rule in this round. The rule's explination includes a strong restriction in regard to forming expressions.
Expressions are symbols composed of symbols.
If there are Co-Wizards, all of them must veto a rule or there will be no effect. Vetoed rules still affect eligibility and style in accord with the Judges ruling on them. Vetoes may be overturned with a proposal to that effect.