In one of the Shodhganga articles, it was stated that
It has been mentioned earlier that the Mahabharata was in the custody
of the Brahmins of the Bhrgu race. The epic got its vast size even
before the rise and development of the Gupta reign. The tendency to
enlarge the Mahabharata text was still continued by these Brahmins.
In their own interest, these Brahmins wrote the Puranas and rearranged
them.
It is a notable fact that Lord Krishna had not a prominent role
to play even in the first connected epic narrative.
The studies of Sri V.S. Sukthankar reveal the fact that the very
first stanza 'नारायणं नमस्कृत्य" etc. is a later
interpolation to the Mahabharata, a pure Vaishnava composition.
We have to remember that Sri V.S. Sukthankar belongs to the next generation to (Late) Sri K. M. Ganguli, and thus might have read the articles of the latter.
We should revisit the words of (Late) Sri K. M. Ganguli on Anusasana Parva:
The Anusasana Parva, like the Santi Parva before it, is a long
hiatus in the narrative of the Mahabharata. Set on the deathbed of
Bhishma, the Anusasana Parva is a huge treasure-trove of Brahmanic
lore.
Why should there be a long hiatus in the narrative of the Mahabharata, unless it was made so by later day scholars?
Further, the Vishnu Sahasranama is a part of Anusasana Parva.
If we remember the observations of Sri V.S. Sukthankar - the very first stanza 'नारायणं नमस्कृत्य" etc. is a later interpolation to the Mahabharata, a pure Vaishnava composition - we can infer that the Anusasana Parva was included at a later date just to include Brahmanic lore and Vishnu Sahasranāma and to project Vishnu as SUPREME GOD.
Thus, Anusasana Parva is an interpolation, as observed by (Late) Sri K. M. Ganguli.