It wasn't just "a thing among Japanese monks" - it's an example of how Japan wasn't quite as Buddhist as one are lead to believe.That's not what I've seen. Buddhism by the Edo period had deeply penetrated all walks of Japanese society. Just because Neo-Confucianism was a thing among Japanese monks doesn't mean they weren't deeply Buddhist: that's like arguing that Europe wasn't deeply Christian because of Enlightenment-era philosophy.
The European equivalent would go something like:
Churchmen arguing morality, proper behaviour and how society should work by referencing 13th century interpretations of
Roman law rather than church doctrine and theologians.
The temporal nobility and actual rulers referencing the same interpretations of Roman law, while occasionally glancing at Machiavelli and
looking to chivalric romances for guidance on how to be proper Knights.
Meanwhile the urban classes have started doing their own thing, combining the juicy bits of Calvinism with concepts from Roman law
(like pater familias and caveat emptor) and bits and pieces from other Christian denominations and folk traditions.*
BUT!
They all go to church on sunday, get baptised and confirmed, learn the catechism, and all the other things expected of good Christians.
WHILE ALSO
Paying respect to the local folk saints and any unreconstructed pagan gods that may still be around, and whose shrines can often
be found on church grounds and being tended to by the local clergyman.
So, deeply Christian, but not only Christian and them being Christians doesn't explain everything.
*The Japanese original here is Shingaku, which is among the things people point to as part of the cultural foundations of Japan's industrialization.
What I have read about it is admittedly a little unclear on the metaphysics, but it tends to come across as more worldly and practical, and
not being very much into the "everything is suffering"-bit of Buddhism.