Thanks for the kind words!
Weather Underground makes a fair amount of information available via the web and its own app(s) with respect to air quality, but--due to its own licensing agreement to use this data--is unable to make it available via the API. There's been other requests for it but alas, it's not there to share.
I took a look at the reference URL from the link you posted and it's not clear to me how the API is accessed. I poked around their site, but didn't see any references to the API or documentation, either. I wonder if it may be behind the registration wall (I didn't register) or if someone reverse engineered it. I also found a package for interacting with the API using Python, but the package is Python 3 only. So it looks like getting the data into Indigo would require a ground-up implementation.
I think this is a non-starter for the WUnderground plugin, but you might be able to find someone here that's willing to write a script or simple plugin. Good luck with your quest!