The Short Version
The evidence supports that China has expressed diplomatic sympathy for Iran's sovereignty and historically helped Iran evade sanctions, but falls short of confirming active support "in its conflict with the United States" as the claim implies. The most authoritative independent source (USCC) notes China's official support after strikes has been largely limited to diplomatic statements. Allegations of military aid rest on unverified reporting and hedged statements, while China's own contemporaneous messaging emphasizes mediation and de-escalation.