“But why do you assume that thousands of people saying ??? ???? ??? ???? would be a z’chus for them?”
Saying ?? ???? ??? ???? is a marvelous endeavor. But, the Jewish custom is to say ???? ???? on the yahrzeit. Deviating from that custom because some atheists decided so is not a very good justification. Kaddish on the yahrzeit elevates the neshama of the niftar; Kaddish not on the yahrzeit, even if doesn’t harm the neshama, doesn’t elevate the neshama.
Regarding gentile customs: they do several different things on this day. Sirens, wreaths and flags are a gentile custom. As a sop to the religious and traditional citizens, at some of the ceremonies, an IDF chazzan recites kaddish and (sometimes) Kel Moleh. So even the Jewish customs are changed to one unified, national day of remembrance instead of saying Kaddish on the day the particular soldier was niftar. Is it better than nothing? I cannot say, but the flavor of the whole ritual is goyishe.