Apparently Markelle Fultz (and Isaiah Whitehead, Bam Adebayo, former Philadelphia 76ers center Nerlens Noel, as well as countless others) received money from ASM Sports agency while they were still technically amateurs. ASM, run by former NBA agent Andy Miller, supposedly paid the players money early in their careers in an attempt to get them to sign with the agency when permissible.
The debate over whether college players should be paid is a contentious one, to be sure, but this is not the place for such a debate. Whether or not they should be paid is indeed open for discussion, but the fact is that these players were paid, in a sense, despite it being illegal.