An example of a skill that correctly lists it's attunement:
The warhorn 5th slot Earth skill, Dust Storm ( https://api.guildwars2.com/v2/skills/30336 ), does have the attunement tag included in it's api entry.
Where as, the focus 5th slot Earth skill, Obsidian Flesh ( https://api.guildwars2.com/v2/skills/5521 ), does not have any indication that it requires the Earth attunement?
There are more than a small number of other Elementalist skills too which have not indication that they require a specific attunement. I haven't taken the time to check all of them, but it seems like most, or all, of the offhand weapon skills are unlabeled, with the exception of those added by the Elite Spec (warhorn).
But all of the focus skills I checked had no attunement tag, and neither did the couple of dagger offhand skills I checked too. (like Churn Earth: https://api.guildwars2.com/v2/skills/5522 )
I'm new to messing with the API and have just stumbled into this, please point out if I missed something obvious, or if this actually does seem to be a bug?
Thank you for your time.