Not sure i would 100% agree with this- Every property added to a Wikidata item is supposed to be referenced with a source. While it is possible to enter things without a source, most people, in particular ones who do bulk imports do provide it.
Now in some cases it is circular - Wikipedia is often sited as a source to populate Wikidata, and in some cases it seems silly (is it really necessary to site iNaturalist as the source of the iNaturalist taxon ID ?)