Arguably they think they do and have good reason for thinking so even if it misses the mark. The main lesson they learned in Warlords was that casual players do not want to run out of things to do. They're not wrong even if that's not the full story.
We can argue forever about the quality of the things they have created to solve that problem but starting with world quests and going on from there the non-instanced non-PVP piece of the game has had stuff to do. Think back to Ion's presentation in Legion and one of the early things he emphasized was that
players would be busy and have things to do.
A lot of what they do now isn't that much to my personal liking but some of the social groups I read (especially on Facebook) are busy with requests about how to do this and that. For the most part they are not complaining. We really don't understand how many players are weekend or only-play-occasionally people but as one of those—I play at most twice a week—keeping caught up, being on the never-ending "treadmill of gear", and anything that is thought of as grindy are things of the past. The game is basically OK for us. There are regular and heroic dungeons, there's LFR and there is whatever there is to do in a zone. This patch apparently is about collecting pets and mounts.
Again, I'd like to see something a bit more interesting that long lists of things to collect but a lot of people enjoy that and they're not wrong to do so. So for me the question of whether or not WoW's developers understand what a casual player is, I think they do although it's an incomplete understanding and even those of us who don't play much recognize when systems are used over and over with minimal amounts of moving deck chairs around.
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Argorwal: I'm casual as fuck and I've been here for over 10 years now. There are others although forums no longer offer much if any useful help (see class forums here). That can be found on Facebook and other social media if one knows where to look. Icy Veins does a decent job of getting people started on playing their class as well. So in one sense you're correct. Casuals mostly don't post here because really, who needs the abuse and there's nothing much here that's helpful anyway.