Reb Moshe is explaining the Rambam Talmud Torah, not Shmita.
Unless you want it to be a stira, you have to explain like the Radba”z or similar (e.g. Maaseh Rokach, who is quoted in the Mafteach as explaining that the Ramba”m in shmita is referring to someone who has enough on his own, or is willing to suffer dire poverty).