Then why not say it? Why not mention bases at all in the official announcement? Why have a "...?" at the end of "shape of things to come"?
Square bases are synonymous with Fantasy Battle, so it's a valid way of teasing things. But since this is a different game called The Old World, it could be they just wanted to evoke the lore and feel of the setting, instead of setting the gameplay in stone.
What has been confirmed is that Old World is to Age of Sigmar, as Horus Heresy is to 40k. A source of background for the lore, sure, but you can also easily use many HH figures in 40k. They're not rival products, like The Old World would be with square bases. Makes no sense to bring different bases, IF the scale is same.
But maybe I'm speculating in vain, and RuneBrush actually heard the words.