One must not read Hebrew prepositions in a manner that they have an exact corresponding English preposition. Sometimes a preposition and adverb can share the same word, a phenomenon found both in English and in Hebrew.
[על כן] = upon the fact, indeed.
Most of the time, indeed would imply therefore.
But for this case, the meaning of indeed is actually indeed indeed.
The verse actually says, Make sure you reached indeed the city called Zoar. Unless you reached a city indeed called Zoar, I cannot help you.
מהר המלט שמה
Quickly flee there
כי לא אוכל לעשות דבר
As not am I able to do anything
עד באך שמה על כן קרא שם
until you reach there indeed named
the city Zoar .
Hrrmmphh ... Bible translators ! ! ! Stop treating as though every Hebrew preposition is perfectly aligned with an English/Greek preposition. Or not realising when a clause is prepositional or adverbial.