I didn't find it surprising and that's what I consider a mistaken assumption, as well.
The harbour seals in the lower river and estuary are the species that take smolts the most - not the sea lions. A good chunk of the sea lions diet is spawning adult herring during herring spawning time - as you pointed out - but that is also in late March to mid April (typically) earlier than smolts come-out., as well.
