For some brewers, the answer was no, not allowed. Others, however, saw no reason not to refill jugs that tour patrons bought elsewhere.
There are some stipulations to the ABC's guidance: Growlers must bear the government warning required on all alcoholic beverage containers, and the tops of the growlers must be sealed. (Breweries have been taping the tops to seal them. There are some additional requirements that say any label has to accurately reflect the content of the container; brewers already do that.)
It may not seem like a big thing, but the ABC director weighing in on the topic is relevant to consumers, especially now that brewery tours and tasting rooms have taken on larger roles for craft brewers.
For one thing, it means that when you're standing at the brewery, holding your growler, you don't have to worry about where the jug came from, so long as it bears the government warning. And there's more than a little bit of importance for craft brewers, especially the under-7 barrel breweries, who can save cash by dialing back on how much new growler glass they keep in stock, knowing they can refill practically any jug a patron brings in.
However, you should still check with the brewery you're touring to find out their policy regarding growlers from other breweries. For the sake of branding, some breweries may limit refills to jugs that came from them and have their logos.