Just from skimming the design doc, it appears that if your clock skew exceeds the maximum bound, it would break linearizability. I haven't parsed through all the details of their SSI implementation, but it appears that even with arbitrary skew they would still enforce serializable transactions. However, it appears that performance under high skew would drop off dramatically.

Without a global clock you basically have to give up uncontended snapshot reads and linearizability for cross-shard transactions. That would be a completely different system from spanner and cockroachdb.