You say the most important question is
can an open source project include the library and still stay open source?
IANAL/IANYL, but as I read it, no. Firstly, section 2.1.4(b) of the licence specifies that
Customer may not ... modify any Licensed Product, or create any derivative work of any Licensed Product
s2.1.5(a) allows you to modify it under certain circumstances, but then says you can't redistribute the modified version. Since our accepted definition of open source says in s3 that
The license must allow modifications and derived works, and must allow them to be distributed under the same terms as the license of the original software.
You could not combine GoJS and any piece of free software into a derivative work, and redistribute it as a free project. I can't square the maintainer's words quoted above with the licence text, and the licence text is likely to have the final word on the subject, but even if we take his/her words at face value, your combined project would be non-free as users would be required to buy a licence for GoJS.