EMIT
I did check the FCTM and it says as you quoted. However what I stated is from FCOM DSC-22_30-70-50 P 5/12. I reproduce the reference below:
The aircraft may deviate from the DES profile while DES mode is engaged if:
‐ Unexpected wind conditions are encountered
‐ Anti-icing is turned on
‐ The lateral flight plan is changed.
So I guess it adds only a small amount as buffer and not the full required thrust. You can make your own opinion.