The Yalkut Shimoni explains that Moshe Rabeinu’s statement, “Hashem will do battle for you”, was not only referring to that point in time at the Yam Suf, but rather Hashem will fight for us forever.
The Meshech Chochma adds, that Klal Yisrael at that time were able to open their mouths and tell Hashem that he is responsible to save them, since he took them out of Egypt. However there will come a time when Klal Yisrael will sin and will not be able to open their mouths to tell Hashem that he is responsibe to save them. That time is referred to in this Pasuk, ואתם תחרישון, is to be understood when you will be silent. And still ה ילחם לכם, Hashem will do battle for us.
Hashem please fight for us now. Although we didn’t keep the rules of corona and we cannot demand that you save us. May you have mercy on us anyway. ה ילחם לכם ואתם תחרישון