Most likely because they are not yet sure whether or not it was a fluke and/or whether or not it would have unforeseen long term impacts and/or they identified other issues they need to address. E.g. the article pointed out that a lot of the increase seemed to be down to fewer meetings. Which presumably raises the question of 1) whether that's sustainable (were those meetings really meetings that could just be cut?) and 2) if it is sustainable, maybe they think they can achieve a bigger productivity increase by slashing meetings and not cutting working hours.

It also seems like one of those things that are near impossible to reverse if you make them the norm and things goes sideways, so I can understand they want to be cautious.