Sorry I worded that badly - I meant that I was trying to explain it.
gmic -negate is 1-x, but accounting for the range of values (subtract min, subtract from new max).
My point is the whole formula boils down to using a reciprocal to invert, the rest is just setting gamma as you might normally. The article/formula is just a very long winded way of saying ‘use reciprocal for film negative’.