The Pope didn't "delegate" exclusive dispensing authority by law in TC. He said the DDW & DICLSAL exercise authority in observing TC. That's not an exclusive reservation of dispensations. Such a reservation would need to be proven explicit in the law.
If not explicit in TC, it'd need to be reserved by new law or a legitimately promulgated authentic interpretation. The DDW isn't a lawgiver & hasn't been entrusted to authentically interpret TC. Regulating observance doesn't include legislatively binding authentic interpretation.