This is not correct. The baseIri needed here is the one of the target site (e.g., "https://en.wikipedia.org/wiki/" for English Wikipedia. The associations sitekey->baseIri are not stable and may be specific to each Wikibase site. So you will need to get this information from an object that can find the Iri (if any) for the sitekey. We can leave this open for now and think about how to best do this -- the alternative would be to have the factory use such an object to resolve the key. We will probably start with a hardcoded default resolution and figure out where to find this information later on (it should be in the sitelinks table that is exported, I hope, with the dumps).