I would agree that there is no need to sweat not having pursued the ExNorms more on the SB5. As on the WISC, they are most likely to add information when learners obtain multiple 19 scaled scores.
I should note, though, that not all evaluators have access to the ExNorms, as they are in a sold-separately interpretive supplement, with which not all schools will have resourced their psychologists. Private psychs may also choose not to expend the resources on it, since the ExNorms are probably the most concretely useful aspect of the supplement, but are only going to be referenced with regard to a tiny percentage of clients. The WISC, in contrast, has freely-downloadable reference tables for the extended norms, which makes it a bit more user-friendly in that regard.
As to what would have been necessary for the ExNorms: as long as the examiner was following standard basal/ceiling/discontinue rules, all of the raw score data should have been obtained. It's the transformation to scaled/standard scores that would be different. The issue typically is that some examiners discontinue as soon as the student has reached the maximum scaled score, even if they have not either triggered discontinue rules or completed the last available item on the subtest. (In some cases, very young children may have been administered the Early Years edition of the SB5, which lacks the highest-level items, and is specifically not recommended for GT. Obviously, not only would ExNorms be not obtainable using the EY, but even good general scaled/standard scores would not.)
Unless one of the caveat situations listed above pertains to your DC's case, it is unlikely that the ExNorms would have made a substantial difference in the composite scores.
