Why is «جاء زيد وهو يضحك» not a direct counterexample to the rule on affirmative imperfect circumstances?

Supplementary Practice 1: Applied Analysis of Faṣl, Waṣl, Ījāz and Iṭnāb

• «يضحك» is a past-tense verb
• The wāw introduces a nominal clause beginning with «هو»
• «هو» is a conjunction
• Wāw is always meaningless