You are confusing two things.
The paper you cite shows how NMN already in the blood can get into cells.
The paper I'm responding to shows that NMN taken vial oral route doesn't get into the blood but is instead converted in the liver.
So if NMN is taken via injection or sub-lingual, then it should get into the blood and then from the blood into the cells.
This goes for NR as well.
Apparently you didn't read this paper https://www.nature.c...55-018-0009-4. In this paper they point out that the researchers in the paper you reference didn't handle the NMN blood samples correctly and got faulty readings.
Two very interesting findings were: "These results clearly demonstrate that Slc12a8 specifically transports NMN, but not NR, in the order of minutes."
and "We have previously shown that NMN is absorbed from the gut into blood circulation within 2–3 min and transported into tissues within 10–30 min (refs 5,15). NMN is then immediately utilized for NAD+biosynthesis, significantly increasing NAD+ content in tissues over 60 min."