Washington's state law imposed many other requirements that went above and beyond agreed-upon requirements in the international agreements negotiated by the federal government. Are the Washington state requirements preempted by federal law?

Yes, Washington state requirements are preempted by federal law.

Explanation:

When Congress uses its power under the 'commerce clause', it can mention that it agrees to have an exclusive regulatory authority. States cannot pass laws that intervene the laws of federal government, though local and state standards are more strict. So, if stricter standards have been imposed, then federal government cannot stop the state from setting those stricter standards.

In this case, the Washington state requirements are not preempted by federal law, even though state requirements are different from the federal one. But for the general ones, Washington state requirements can be preempted by federal law.