If we're in nitpicking mode, then seal script wasn't excusively used in
the Qin either, but also before and after.

It's mostly preserved as a calligraphy style nowadays. I don't,
however, think that the words on this pot is strictly seal -- I don't
know seal script that well, but it might be words that are styled in
the seal script style, but would in fact be incorrect based on the
original seal script way to write these words. That's just a guess.