The race to replace Redis

What do you mean by "purely as a matter of copyright law"? The libslice license agreement states that to copy libslice, you must do X, Y and Z. Z is releasing the source code of libdice under AGPL. If you don't do that, you have no right to run slicendiceserv on the network as you copied libslice without permission.

There is no need for libslice and libdice to have any relationship, for this to apply. It is as if I make a contract with you: I give you $5, and you refuse to paint John's house. We can enter that contract even if neither of us is a house painter and even if neither of us know where John lives. There's no asking "as a matter of house painting law, can the $5 be related to the house painting?"