"what makes a song NS"

Well, my answer to that is that it's quite simple really ... a song is NS if it has ever been a popular spin at "Northern Soul" events.
I suspect that you are making the common mistake of thinking that "Northern Soul" is a particular type of music when I think that it is really a rather insular, and to outsiders rather mysterious, "scene" consisting of various events where people get together to listen to, dance to, and discuss or even buy what is being played. The people who attend these events think of themselves as being musically aware and expect to hear music that falls within certain parameters ... a record should be midtempo or uptempo and have a regular rhythm prominent throughout most of the song (though the occasional break is allowed and often welcomed), it should have a '60s (or early '70s) "feel" and ideally any vocals and instrumentation should have a soulful sound.
Then we have the whole "rarity" factor at the "Northern Soul" events ... ideally anything played shouldn't be too well known to the outside world ... the whole scene has a big collectors element, those attending are often keen record collectors and there are likely to be stalls there hidden away in corners with record dealers selling old 45s. Rarity is important and if playing a popular oldie DJs will get "brownie points" by playing the tune off a 7" copy on the original label or from a particularly rare issue, they will usually get "Null Points" by playing a tune off a bootleg 7", a CD, an MP3 or something similar.
Many people will have travelled a long way to attend ... back in the '70s they wouldn't have wanted to hear something that they could have heard three times a day on their local radio station ... that attitude remains at many venues where the emphasis is on "rarity", with DJs seeking out ever more obscure tracks to spin as "exclusives".
So really we have a chicken and egg situation where particular recordings become "Northern Soul" by being played at NS events and events become "Northern Soul" events by playing NS records.

As to your Anita Baker oddity, it does have a regular midtempo rhythm throughout most of the tune which will appeal to the dancers and it's not very well known.
It also has a similar feel to the extremely popular "That's Not Love" by Holly St James which came in at #153 in the 2000 NS500 and #52 in the 2022 NS500.
Any more questions?
Roger