WWDC is a bit different, though, a much more generalized presentation than the annual specific iPhone event and keynote; at the latter, iPhone event, they usually spend time talking about iOS and all the wonderful new features and functionality the newest update will offer and they spend time talking about the iPhone and the various models, etc., plus there is always the statistical bit thrown in there, too, along with demos of software that will work nicely with the new iPhones (or iPads) and the latest version of iOS. Oh, and maybe a fun opening sequence video like they've been doing recently. This time iPad Pros will probably be part of the presentation, too, which they haven't done in the past, as they were updated at different times of the year. Just my thoughts on this. By the time they get through all that, really not enough time to talk about a new version of the MacBook Air! Of course I suppose they could toss it in there right at the end, pulling one of Steve's famous "....and One More Thing...."
That is true they will probably spend some of the time showing off new software features for the iPad Pro’s and iPhone’s, what those features will be we don’t know yet. The iPad Pro was introduced at the September keynote in 2015, last year it was WWDC and this year it will probably be at the September event, it does seem to be up and down.
This years iPad Pro is going to be a redesign as well with Face ID so it might take longer, you could be right and they might not announce the new Air or updated MacBook, they might hold an October event or even a silent update.