A! Before the interview was published people actually did think the series ended.
When the Tankōbon for volume 5 was released earlier the same year, the Obi (which is a strip of paper looped around japanese published books) had a line that read “The end of summer that never ends”. This made people think the manga actually ended; and it doesn't help the fact that Azuma years later stated that the beach chapter was a climax he had in mind when he started making the series, which imo does have a natural end to it.
Yotsubato also, at the time, was running for as long as Azumanga had. So there's that.
Then again, people said the same for volume 12's camping chapters so maybe we've always been contemplating the inevitable…