What is this paste I got from boiling down coconut "whey"?
"Coconut Sugar" is actually made from the sap of the plant, not the nut. Basically, you made a type of "coconut whey paste".  Since it lacked the fiber and most of the oils it was viscous and sticky.  Also, since it included "milk" from the "testa" (Peel of the coconut) it was sour.  

So you reduced the whey down to its: solids + sugar + some testa.  As I said sugar is made from the sap of the tree so I don't know of anyone who has found a use for what you have created.  I would be very curious what you end up with if you peel the coconut before the process.  I would bet it wouldn't be as sour.

With a little more work you are well on your way to a "Coconut Protein Bar" recipe! lol

References:
https://link.springer.com/article/10.1007/s11746-014-2447-9
https://www.researchgate.net/publication/272590676_Composition_of_Coconut_Testa_Coconut_Kernel_and_its_Oil
https://foodcomas.com/2013/03/20/kerisik-coconut-paste/
https://www.healthline.com/nutrition/coconut-sugar