That article about the 2016's signing with Miami probably doesn't give us all the details of the situation. It's not permissible to sign a school's athletic scholarship offer prior to the NLI signing date for that sport. In addition, it's not possible for a school to issue a National Letter of Intent unless the NLI is accompanied by a school's athletic scholarship agreement.
Here's what probably actually happened:
Division I Bylaw 13.9.2.2 Written Offer of Aid Before Signing Date. Before August 1 of a prospective student-athlete's senior year in high school, an institution shall not, directly or indirectly, provide a written offer of athletically related financial aid or indicate in writing to the prospective student-athlete that an athletically related grant-in-aid will be offered by the institution. On or after August 1 of a prospective student-athlete’s senior year in high school, an institution may indicate in writing to a prospective student-athlete that an athletically related grant-in-aid will be offered by the institution; however, the institution may not permit the prospective student-athlete to sign a form indicating his or her acceptance of such an award before the initial signing date in that sport in the National Letter of Intent program.