The FCOM2 states that N1 and N2 overspeed protection is provided both in normal and alternate mode.
As far as I understand you can still overboost (too high internal pressures) in alternate mode. In soft alternate engine parameters (e.g. limiting N1) are defined from the last valid environmental conditions. So if the air would get any colder/more dense you could potentially cause damage if you would set max N1.