The only concern is that Wikidata is a single wiki and not all users on a given wiki will be concerned (except for adding interwiki links but technically ll is done on this subject). The rest is about data that will be used by tricky templates and whose readers would be more technical users than usual.
So if you add things about Wikidata, this should be generic enough to interest most people (notably contributors writing articles) and not just developers of tricky templates using Wikidata; such things could include some technical issues to discuss about translation and localisation of this data, creation of Wikidata user groups interested in some projects (such as adding reference IDs or supporting the integration of some data related to GLAM collections).
If this is just to discuss about migration of ontologies, and this does not require a large participation (if this can be done essentially by bots), then I think that these technical news should remain on Wikidata (and use a separate distribution for real developers of Wikidata): nothing forbids duplicating an announcement on both letters, but typically the annoucement in Tech News should be much shorter than the Wikidata technical news. If the tech news is just about the change of support of specific extensions used only by Wikidata, this should not be in Wikimedia Wiki Tech news which are really cross-project and should interest most users of Wikimedia sites contributing pages (and that's why the VisualEditor takes a good place in these Wikimedia Tech News).
Note also that most news related to Meta-Wiki and policies (or financement campaigns, or elections in the WMF comitees) are handled in a separate messaging system (currently using the old "Goings-on" system that should evolve later after the Main Page will be really fully translatable with the Translate tool) : they are also announced not primarily by MassMessage but by global site notifications (but some users may opt to receive these Wikimedia notifications by mail, notably for things like new global policies or guidelines, or opening of a new Wikimedia content project).