I was at AMS at the time. Patches of black ice were indeed to be found on the taxiways and the ATIS carried the usual warnings.
Doesn't really matter what reasonable speed you are doing if you happen to try to turn on a patch of black ice you aren't going to. Nobodies fault - just unlucky.
EZY taxi limits are 25kts and this is monitiored at all times by the computer and a 3kt exceedence results in tripping the safety flagging system. I don't think you'll find EZY have a taxi speed problem - in fact they are frequently frustrating to be behind as they crawl along like geriatric farmers on a Sunday I find. Certainly the case at Gatwick.
Regards,