I re-read the letter again and I think I might have misread it.
It reads (EMPHASIS MINE ):
“If God, because of a righteous man withheld judgment on Sodom, OR because of 120,000 who didn’t know their left hand from their right”
OK, God TEMPORARILY withheld judgment from Sodom, the OR in his letter tells me that he was referring to Ninevah but did not name the city explicitly.
Yes, Abraham first plead on behalf of Sodom, asking the Lord repeatedly if he would spare Sodom if he found 50, then 45, the 40 and so on all the way to “What if only ten can be found there?”. Then we read the story of the destruction of Sodom, but that destruction only came after Lot vacated the city and he was told “I will not overthrow the town you speak of. But flee there quickly, because I cannot do anything until you reach it.”