This is just a guess on my part, but I believe it may have to do with the differences between 1973 and 1983 Canon Law regarding clerics.
Under the 1917 law, a man became a cleric when he received first tonsure when he entered the minor orders. However, under the 1983 law, a man only becomes a cleric upon his ordination to the diaconate (the Novus Order "ministries" of reader and acolyte are somewhat analogous to the traditional minor orders, but the 1983 law is clear that they are not clerics).
So, if the SSPX were to become regularized with Rome, anyone admitted to the minor orders would not be considered clerics.