Funny but I don't read that article as saying they aren't in favour of it, more saying that there are other ways that can be used as well
I'm speaking to the various ads which W3W were running showing people in distress using the service. The point made in the article is that W3W isn't a panacea, and that people shouldn't head out into the wilderness depending on it, which the advertising campaign strongly hinted at.
One example told the story of a climber who was injured in a fall on a high mountain - it may have been Everest, saying how he was rescued by contacting the rescuers (99% sure it was in WhatsApp), who told him to download the app and send his location using W3W coordinates. Somehow it seemed to completely bypass everyone that they were introducing unnecessary complication to the process (the location button in WhatsApp!)
For completeness (and hopefully to end this side discussion) I have zero problem with the concept behind W3W, my issue is that they have oversold what they do (use a simple algorithm to convert geolocation data provided by the mobile device to another set of coordinates specified in a different system), and that they have kept the algorithm and database proprietary.
At its root, they don't do anything more fancy than convert lat/long to OS grid reference.
The fact that both the database and algorithm are secret gives them the ability (if not the intention) to lock people into their system, something that we should all be wary of.
And which emergency services have said they prefer post codes?
I will need to look for it, but I read an article that said at least one service preferred postcodes since that's what satnav systems are set up to use, probably more of an issue in urban rather than rural areas (since the postcodes cover a smaller area).