Answer

A good question. Let me clarify for the readers. In the lecture I cited the words of the author of Dvar Avraham in the Kovno Ghetto, who instructed the Jews there to pay the ransom that was demanded of them for the Jews of Jonava, even though that money was like a flask of water for them and put them in mortal danger. At first glance, the Jewish law follows Rabbi Akiva: the owner of the flask should drink it and not share it with his fellow. His argument was that already now, in the ghetto, they were not considered truly alive, and perhaps specifically by virtue of the commandment, the Holy One, blessed be He, would save them to life.
Now Yitzhak asks: why don’t we say the same thing in the case of two people walking in the desert? Maybe by virtue of the commandment, the Holy One, blessed be He, will save them.
The distinction is simple. I should first say that there is no certainty that the Holy One, blessed be He, will save someone by virtue of the commandment. This is not an assessment but a hope. It is the only chance, even if it is small. Now, in the case of the flask, the owner of the flask will certainly survive with it. So he is forbidden (or at least not obligated) to endanger his life in order to save others. As for the chance that both will be saved by the Holy One, blessed be He, we do not rely on a miracle. Therefore here his life takes precedence. But in the case of the Kovno Ghetto, even the owner of the flask probably will not survive, and that is so even if he does not share it with others. His only chance of survival, even if small, is that he perform the commandment and God will help him.
This is similar to what I wrote in my article about Siamese twins. When gentiles besiege a city and demand, “Hand over one of you, otherwise we will kill all of you.” Strictly speaking, it is permitted to hand over one person (and to hold a lottery to determine who it will be). Although it is forbidden to kill one person to save another, here the alternative is that both will die, including the one who is handed over. So they hold a lottery that sends him to die, but before his lot was drawn, that lottery was also his only chance to survive. Only because of desecration of God’s name does Jewish law forbid this (see Maimonides, Foundations of the Torah 5:5, and my above-mentioned article). See there for implications. The same applies in our case. The alternative is that both the Jews of Kovno and the Jews of Jonava will die. Therefore, in such a case, they share the flask, because that gives them their only chance to survive.