The period of time they draw on is about half as long.
Many classic rock stations play material from the early 70's to mid 90's these days. Many alternative stations play material from the Seattle grunge era (early 90's) to today, with a few choice older cuts (e.g. Beastie Boys) thrown in for good measure. So, total number of years featured in regularly played material is about the same.
That being said, I definitely would agree that classic rock possesses a significantly higher number of playable songs, but it appears WDZH will likely wrap up its A to Z marathon at about the 36 hour mark. That seems excessively short. Compare that to classic rock, where such marathons often take up to a full week. Should there be a duration disparity? Sure. Should the disparity be that wide? In my view, probably not.
The KROQ and WDZH marathons seem to be playing very little post grunge, little if any Lillith Fair era material, and in the case of WDZH, nearly zero industrial rock (I saw a couple Nine Inch Nails songs - nothing further) and no alternative metal.
I was far more impressed by the A to Z playlist featured on 91X in San Diego earlier this year. Usually, stations do A to Z to showcase the depth of their music catalogs. 91X definitely succeeded in that regard. In the case of KROQ and WDZH, I'm underwhelmed.
I will say the all 90's weekends currently airing on three of Cumulus' alternative stations - each with substantially different playlists - are much more to my personal liking. Check out WZRH New Orleans, WEDG Buffalo and KCJK Kansas City. I really like the changes Jason Ginty has made to WZRH in New Orleans.