That's quite obvious, isn't it? No one else has won 15 slams. You can't really compare Laver's total "slam" numbers (amateur + pro + open) to Open Era players.
Besides, he already has the record at 3x4.
Why would that be obvious? There ARE other permutations possible with 15/16 slams, are they not? like 7 Wimbys, 5 USO's, and 3 AO's and 1 FO for instance, so even though numerically no-one else could have done it, Fed may not achieve it either
PS: Besides which, no matter how "obvious" a record may be, it is still normal practice to note it "for the record"; being obvious is not usually a rationale for omission.