The second possible explanation - that previously undiagosed adults are now finally being diagnosed and treated - solidly fits the medicl facts that we know.

ADHD is highly heritable, and is a life-long difference in brain structure.  Around 11% of North American children are diagnosed with ADHD (and therefore have almost certainly inherited ADHD from one or both parents).  On the other hand, in 2015, roughly 3% of adults were diagnosed, by which "back of a napkin" math we can surmise that roughly 7% of adults had ADHD but were not diagnosed. By 2025 this percentage of diagnosed adults was just oveer 6%. Which suggests that around 4% of adults can be expected to have ADHD but still be undiagnosed.

Our medical understanding of ADHD has grown vastly in the intervening decade.  Likewise our understanding of medications and ADHD.  

As such, finally starting to make a dent in the "back catalogue" of undiagnosed-but-ADHD-positive adults could be expected. At some point, it might be expected that approximately 10% of children and also approximately 10% of adults will be diagnosed with ADHD.  Because, again, it is strongly heritable, and lifelong.  Adults now being finally diagnosed (sometimes because their children are being diagnosed, and it is now recognized as heritable) always had it, as did the undiagnosed adults that they in turn inherited it from.

The specific data is certainly welcome.