).25 pounds of salt per 100 gallons is actually only 0.03% salt, which does protect koi from nitrite poisoning even when nitrite is as much as 65 ppm.
The rule of thumb when protecting fish from nitrite poisoning is to have chloride ion present at at least 10 times the nitrite level to avoid a significant nitrite uptake through the gills.
So let's assume that nitrite levels during cycling of a new pond hits 10 to 20 ppm levels, which is often the actual situation.
Let's assume worse case, 20 ppm nitrite for a couple of weeks.
10 times 20 ppm is 200 ppm chloride ion needed, another way to write 200 ppm is 0.02%. So we need 0.02% chloride concentration, and sodium chloride is 60% salt, so we need 0.02%/0.6 = 0.033% salt to protect really well from 20 ppm nitrite levels.
0.033% salt is achieved by adding 0.27 pounds salt per 100 gallons of water, or 2.7 pounds salt per 1000 gallons of water.
If no salt is added, we cannot safely assume there is sufficient chloride ions in the water to protect from nitrite poisoning.
So my preference is to tell ponders to add a pound of salt per 100 gallons during cycling of a new pond to protect from nitrite poisoning, which is enough to protect from nitrite poisoning even if the nitrite gets to 100 ppm (which almost never happens anyway).
I hope that answers the question in sufficient detail.
Like I said in the other public post, the aquaculture research paper I quoted showed that 0.25 pounds of salt per 100 gallons gave complete protection to carp (koi) from 65 ppm maintained nitrite concentration for a few weeks test period. But 0.025 pounds salt per 100 gallons gave deaths of carp at 65 ppm nitrite levels in only 24 hours.
So do some protection when nitrites are there, but there is no need to add large quantities of salt.