I think it is an attempt to identify the latest date on which a person could be entitled to HB/CTB on a claim made before the NINO requirements came into effect.
Subsections (1A) to (1C) were inserted into s1 of the Administration Act from 1997, but HB Reg 2A disapplied the requirement for HB purposes until it was revoked in September 1999 (and earlier in some pilot areas I think). Because s1(1A) – (1C) is about the process of claiming, and is not concerned with the claimant’s continuing weekly entitlement in the way that income, capital and family circumstances are, anyone who claimed HB under the pre-NINO rules up to August 1999 would not have been required to provide a NINO until perhaps the end of September 2000.
We have been discussing the common-law presumption against retrospectivity, backed up by s16 of the Interpretation Act, in overpayment threads lately. Here is another example of that presumption in action – no-one can be deprived on NINO grounds of HB that they are entitled to for a period before any such requirement existed.
You could put it another way: HB entitlement for any benefit period (remember those?) beginning before September 1999 is not subject to the NINO requirement.