The Algorithmbase is 99.999% of what makes Mathematica useful compared to any language with a decent REPL or notebook setup.
I can't really imagine a use case for the Wolfram Language if I'm not going to use anything listed in http://www.wolfram.com/algorithmbase/
I personally hate programming in Mathematica, but it is interesting and deserving of an open implementation.
I actually believe the Algorithm Base is probably included, but it's definitely anything but clear from the website or any of the press release. The Mathematica website makes them seem like two completely separate unconnected components.
It's really unclear what is included in Wolfram Engine and what is included only in Mathematica... the above was just one obvious example. It would be very nice to have a concise list.
If you look on their web site for Mathematica[1] it has several bullet points that includes the Wolfram Engine. One of the other separate points is "Wolfram Algorithmbase"[2], so presumably all those algorithms are not part of the engine and only available if you buy full Mathematica.
[1] http://www.wolfram.com/mathematica/
[2] http://www.wolfram.com/algorithmbase/