I'm a bit dubious about how this is going to work - is it going to include online catalogues of local archives, libraries, and other organisations, for example? What are they meaning by 'historical sources'?
Sites like the Archives Hub, SCAN (for Scottish archives), both the National Register of Archives (NRA) and the National Register of Archives for Scotland (NRAS) offer something similar, though of course different things are found on each and none are completely comprehensive. I don't think there will ever be a fully comprehensive 'search engine' (and by that, they mean just that - I agree that many results may still be pay per view or subscription sites) because the nature of history and historical sources means that there is so much out there and not enough time, money or people to fully document them.