The UBI is considered an NIT because they are exactly the same policy.
For any given UBI (b), and initial tax rate for all income above the UBI (tb), there is a tax threshhold (hN) and initial tax rate (tN) for an NIT scheme that has exactly the same effect.
Proof:
If (It) is income after tax, and (I) is income before tax, under UBI:
It = I - [I x tb] + b
and under NIT:
It = I - [[I - hN] x tN]
We therefore need to find a solution to:
I - [I x tb] + b = I - [[I - hN] x tN] for all I
Note that if they are equal for all values of I, they must be equal at I = 0. Therefore we can conclude that if there is a solution, it definitely satisfies b = hN x tN (what you get if you let I = 0 in the above equation).
We can substitute this into the above for:
I - [I x tb] + [hN x tN] = I - [[I - hN] x tN]
Rearrange for:
I - [I x tb] + [hN x tN] = I - [I x tN] + [ hN x tN]
Cancel out equivalent terms and you get:
tb = tN
Therefore any UBI scheme has an exact NIT equivalent at tN = tb and hN = b / tb [the only exception is for tb = 0, i.e. a basic income with no income tax at all].
As an example, a NIT with a threshhold of £30,000 and a 50% tax rate has an exact UBI equivalent at a £15,000 basic income and a 50% tax rate. If you have £17,000 income before tax, you finish with £25,500 in both systems. So there is no economic reason to prefer an NIT to a UBI - they do exactly the same thing. The only differences are that a UBI is perhaps moderately easier to administer (you only need to check income when raising tax as opposed to both when raising tax and handing out subsidy, which are typically administered by different institutions) and the NIT is slightly more politically defensible (not "giving money" to rich people).