I have spoken with the maintainer of said library.
Their intention was not to make the software nonfree source-available. They just didn't have a clue what they were doing and edited the language used in the license file incorrectly.
Turns out they were transitioning into a company and their intention with their new license was to ensure that all external contributions assigned their copyright to them.
So in order to help them I suggested to them that they should revert the license back to the original MIT/Expat license text and add a Contributor License Agreement instead.
And they accepted the proposal!
I am personally not a fan of CLAs, but if companies want to go this way, it is a good middle-ground to make sure that at least the software itself now remains free as in freedom, all thanks to me.