In this case, IIUC the swap-restore mechanism just can’t use fragmented files because it runs before filesystem drivers can do their work. So it just gets an offset and probably derives the length from a header stored there, using raw disk accesses to thaw the RAM contents.

In general terms though, I hear disk fragmentation still makes a difference, f.ex. because disk page sizes don‘t match filesystem extent sizes, and disk optimizations like prefetching or even garbage collection work better with contiguous data.