The documentation does not mention it but as far as I know the SORT command is implemented using quick sort. This also explains why the sort is not stable.
Since ABAP is a 4th generation language, it would be pretty weird to not have a SORT command. And an implementation in the kernel will always be faster than anything built in ABAP. Why you need it in your program is a whole different story, I see it misused a lot where the sorting could have been done during SELECT or the sort is only done to remove duplicates that could have been avoided with some smarter SELECTs.