DD supports lattices that allow it to compute at multiple points in time simultaneously. As I understand it, DBSP limits time to one diff at a time. Lalith can correct me if I’m off base on this. :)
I'd say the difference is in the type of transaction isolation guarantees each system provides. DBSP can process multiple diffs in parallel, and when it's done it outputs a single diff that captures the effects of all the input diffs. DD can additionally attribute each output diff to a specific input diff by assigning each input diff and matching output diff a logical timestamp. This has a cost in terms of complexity and runtime overhead, but it allows strong isolation of concurrent transactions.
But as gz09 said, both DD and DBSP are data-parallel architectures that can evaluate queries concurrently on multiple threads or multiple machines.
(lalith here) -- Whatever ryzyk and gz09 said above. :)