Books, again
Vilas:
With regard to your mention of, FCOM says, Descent is calculated with idle thrust (i.e. anti-ice off), let me give you this quote from FCTM:
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The first segment of the descent will always be idle segment until the first altitude constraint is reached.
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This would appear to confirm your assertion.
Continue reading however, the next paragraph:
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The idle segment assumes a given managed speed flown with idle thrust plus a small amount of thrust. This gives some flexibility to keep the aircraft on the
descent path if engine anti-ice is used or if winds vary. This explains THR DES on the FMA.
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Do you now understand my assertion that the descent path is calculated with the assumption that anti-ice is on?