Exactly. Often the shipping weight will be measured as it is shipped (likely because most of these are coming by air). Alibaba and others pass this shipping information on to the customer and would likely notice overly low shipping weights.

Fraudulent sellers will also use a similar method in certain countries where the delivery address is not tracked but weight and delivery status are. They can send a nonsense package to the right country/state, have it appear in China post tracking as delivered, but the fake package was actually went to a random address with a roughly expected weight. If customers try to file claims on sites like aliexpress they'll likely rule in the favor of the seller unless you can somehow get your local post office to provide proof it was never delivered to your specific address.