It is legalism because Christ's Jewishness is based on it, and his Jewishness is absolutely required by the OT for him top be the "Anointed One" (the "Meshiyah," aka "Messiah," aka "Christos").
But in order for him to be the firstborn of redemption (i.e. the firstborn of the matrilinear line) he would have had to be the first to "open the womb" and not just "jump" out, so to say, as if walking through closed doors.
Now, being the firstborn of the Father simply means that he inherits twice as much as the rest. Yet that is a problem as well, since Christ inherited all, since there is no "rest"! So, basically, Jewish legalism were retained where they were needed and discarded or modified where they were a stumbling block. :)
I agree wholly. We are not bound by the Jewish legalisms. When they accord with the natural law we retain them, e.g. we retain and in fact expand (in the sense of the Sermon on the Mount) the Ten Commandments. The dietetic laws, and this tiny literalism that you note, and the entire spirit of legalism we reject.