Hi there, this is a great question, but I think it may be a little hypothetical. First of all, I'm assuming you're talking about North American glaciers.

Seasonal snowmelt certainly is a big part of the western water equation. Seasonal changes and long-term changes to snowpack and the timing of snowmelt are major concerns in the water future of the US Pacific Northwest. With that said, I don't think glacial melt (the melting of much, much older snow that has become ice over decades/centuries) is a significant part of the year-to-year water scene. 

So, while North American glaciers are indeed undergoing change and retreat, and there are significant ecological and other ramifications, drought metrics consider that, as they are not really a big component of the water budget on the scales that we apply to operational drought assessment (weeks to years).

(confession: I'm speaking off the cuff here, and my background is mainly in agricultural drought and mostly east of the Rockies. I haven't done any formal homework to answer this question. If I get a note from someone more glacier-minded or Northwestern than me, I'll make sure to follow my comments up here)

But, hypothetically, yes I could see your point that the retreat (or hypothetical advance) occupies a drought timescale that we didn't consider in the article.

Next drought meeting I go to, we could argue about this for days!

Deke