Why does Maimonides ignore the passage about "one who is engaged in a commandment" (he doesn’t rule anything on it, etc.)?

See Maimonides, Laws of Shofar 6:4:
Those sent on a commandment are exempt from the sukkah both by day and by night. Travelers by day are exempt from the sukkah by day and obligated at night. Travelers by night are exempt from the sukkah at night and obligated by day. City watchmen by day are exempt from the sukkah by day and obligated at night. City watchmen by night are exempt from the sukkah at night and obligated by day. Watchmen of gardens and orchards are exempt both by day and by night, because if the watchman makes a sukkah, the thief will know that the watchman has a fixed place and will come and steal from somewhere else.

That is not a ruling about the general principle of one engaged in a commandment. Besides, Maimonides did not mention the rule that tefillin merchants and the merchants who travel for them are exempt from the commandment.

So what? That itself is his ruling.