I have some sympathy for the inference from the use of specific types of materials of which the object is made. But that inference comes with implicit assumptions about the designer based on prior observations of designs made by humans. If it’s made of plastic, has circuit boards with copper wiring, chips of silicon and so on we’d infer something like humans made it supposing we had somehow become convinced we couldn’t have done it ourselves.
But again this inference is only possible because you have at least a rough model of the designer and it’s capabilities, means, and motives. A purported alien species at a technological level close to ourselves.
Now you just need to actually find something equivalent in biology.