Bamidbar By The Numbers

Last year I wrote a post about Parshas Bamidbar which raised the question of why, out of a population of at least 1.5M (600K adult males, an equal number of women, plus small children) there were only ~23,000 male first born (bechorim). This means that assuming a male child was born first about 50% of the time, only about one in 34 children was a bechor. The answer I proposed was that many Jewish first-born died during the makas bechoros (plague of the first-born), thus leaving relatively few surviving bechorim. A variation, as discussed in the comments, is that the deaths occurred during the plague of choshesh (darkness), as per the midrash that 4/5 of the nation perished at that time, reinterpreting this as 4/5 of the leaders, i.e., the bechorim.

Today I was perusing the Hertz Chumash commentary, which I used to know extremely well, based on reading it every Shabbos of my childhood while bored in shul. But today I noticed an interpretation of his addressing the above issue, which I hadn't remembered seeing before. It is unattributed and reads as follows:

"What is meant is the number of first-born males under twenty years of age at the time of the census. The law did not have retrospective force, so as to include all first-born sons throughout the nation who themselves were fathers or grandfathers at the time."

Has anyone heard this interpretation before? Do you know if there are any Talmudic or other classic sources for this, or is it Rabbi Hertz's own idea?