With respect, you're doing the same thing Emma DeSouza did in the case she lost - arguing the Agreement says what it doesn't and not taking note of what the law states.
Cambridge University published a paper in 2022 which stated the following:
"The British Nationality Act 1981 ensures that anyone born in Northern Ireland to a parent who is settled or is a British citizen automatically becomes a British citizen. It was not altered in light of the 1998 Agreement. The people of Northern Ireland can thus identify as Irish and can assert their entitlement to Irish citizenship, but if they do, they will be treated in domestic law as dual citizens unless they also take steps to renounce British citizenship."
And:
"Because the people of Northern Ireland were automatically presumed to be British citizens, this change meant that they could no longer circumvent minimum income requirements for securing the residency of family members by asserting their Irish citizenship, unless they also renounced their British citizenship."
The Upper Tribunal concluded in DeSouza's case that the Agreement "does not, in fact, involve giving the concept of self-identification the meaning for which the claimant argues".
In other words, if the Belfast Agreement was referring to nationality as DeSouza essentially argued it would have stated so explicitly. But it does not.
The part of the Belfast Agreement I quoted must be considered in its entirety and not in part.
Indeed, the case of Irish national, Lisa Smith, directly contradicts Emma DeSouza's since Smith was essentially arguing she cannot be excluded from British nationality given her parent was born in Belfast! She argued she should be automatically considered to be a dual British and Irish national. Even though she argued she was Irish and had no intention of acquiring British Nationality leading Judge Underhill to say:
"...it does not seem to me disproportionate to treat her in accordance with her chosen status as a foreign national,” Lord Justice Underhill said, adding that it would be “extraordinary” for her to have the right to freely enter the UK as a British citizen without being willing to acquire that status.
“Ms Smith cannot have it both ways. If she wishes to be treated as a British citizen she must accept the obligations that go with that status, including allegiance to the Crown, which is owed by those who are British from birth just as much as by those who acquire nationality subsequently."
Smith’s lawyer argued that this infringes on her rights as per the Belfast Agreement. But Cambridge noted that "The (Special Immigration) Commission nonetheless maintained that the obligation on someone claiming to have entitlement to British citizenship to take an oath and pledge of allegiance ‘is not inconsistent with the rights of those who wish to identify as Irish’ under the 1998 Agreement."
Theyfurther stated that "Smith’s eagerness to assert dual nationality to prevent exclusion highlights how different claimants are drawing upon very different accounts of how nationality law works in the context of Northern Ireland."
The constructive ambiguity of the Belfast Agreement meets the realities of law - on this issue, domestic.
It demonstrates how identity is often contentious and how people bind it up in a manner that suits themselves. For DeSouza, the Belfast Agreement means the UK Government has no right to automatically designate her a British citizen. For Smith, the UK Government should automatically consider her a British national with dual nationality.
Both invoked the Belfast Agreement, but both cannot be equally true. Both women lost their cases against the UK Government.
In conclusion, there is a hence. The Agreement speaks to identity, not nationality, and both Governments agree not to prohibit their respective citizens from acquiring a British, Irish or both sets of passports.