Hill did not think the River Styx, with whom he had only briefly interacted, would care about the Undead.
To that being, these souls did not belong to him in the first place. No matter what antics they got up to, they had nothing to do with him... besides, the Undead definitely would not steal his souls in large numbers.
The River Styx did not even care when the Gods fished believers' souls out of him, which showed just how high his threshold for "large numbers" had to be.
Take this world, for example. No souls had entered the River Styx for tens of thousands of years, and its tributaries had nearly dried up before the old fellow finally reacted at a leisurely pace.
And his resistance had been especially feeble. He had clearly only stretched out a claw to shoo them away; if they left, fine, and if they did not, whatever.
The reason the River Styx had been shaken enough to keep such a close watch on the Undead... was probably Lathander, right?
The fact that Toril's boundless River Styx had once been jammed to the point of madness must have left a deep impression on the old fellow.
Ao restarting the world would not cause that kind of trouble... that guy would stuff most souls into Abeir, which existed in another dimension, for temporary storage, then release them back into Toril once the world had mostly recovered.
What happened with Lathander had been a true accident. More importantly, he had genuinely intended to bring them all back to his Divine Kingdom... The Yellow Springs jokes related to that period were basically all about a lonely elven beauty standing by a stretch of the River Styx, leaning against the bank and gazing into the distance.
No matter how