That is true. But finding out those numbers is a much bigger problem for you or I than it is for a national government that can dedicate full-time research staff to producing sensible estimates.

So a benefit of this policy is that environmentally conscious private citizens no longer need to feebly grapple with such questions. You and I just optimize for personal expense and our own happiness while computing the carbon footprints of various things is the responsibility of institutions that actually have the resources to do it.

If a company is trying to obfuscate you could just have a high base rate you apply to them until they pass an audit to prove that they have an actually low footprint.