After further research, this seem maybe to not be what it seemed at first.

This message seems to be saying SEATED children can't be accomodated on Business Class on the 787, but it doesn't preclude Lap Children.

Which makes it doubly odd that I get this message and it prevents me from making a seat assignment because I have only ticketed a lap infant.

But the operative term seems to be "Infant occupying a seat".

This other URL made it much clearer:

http://www.ana.co.jp/wws/us/e/asw_co.../family/guide/