MTG reactive play denied in MTG Arena
Though not exactly the same, Zareth San's ability is very similar to Ninjutsu, and even referred to here.
702.48c states:

A ninjutsu ability may be activated only while a creature on the
battlefield is unblocked (see rule 509.1h). [...]

And when we follow up on 509.1h, we see that

An attacking creature with one or more creatures declared as blockers
for it becomes a blocked creature; one with no creatures declared as
blockers for it becomes an unblocked creature. This remains unchanged
until the creature is removed from combat, an effect says that it
becomes blocked or unblocked, or the combat phase ends, whichever
comes first. A creature remains blocked even if all the creatures
blocking it are removed from combat.

Since you added that your opponent cast Stomp during the declare attackers step, it wasn't determined yet whether your duelist is blocked or not. Therefore you weren't able use Zareth San's ability.