I don't really think there's enough evidence present yet to tell just how interactive sidekicks will be... And the term 'banter' may mean something more specific to Josh than what it means to players.
Personally I see banter as very specific scripted/responsive interactions between characters within the party while traveling - not necessarily things like responding to the situation or surroundings etc, and completely separate from actual dialogue. For example, the conversation between Tekehu and Maia(?) near the beginning of the stream.
Adding any noticeable number of those for each sidekick (in addition to whatever they likely currently have) would actually be quite a large undertaking. Because it would be expected between them and every other companion and sidekick - so very specific personal interactions with 10* other characters. That kind of work grows exponentially with each additional point of contact.
Like I said, it's not really an assumption we can make at this point, but I believe the sidekicks will do just what he said - be colorful, have some fun lines here and there (maybe some both in and out of dialogue), probably have some personal dialogue trees that the Watcher can delve into, and generally be fun but without much story impact.
Based purely on what he said, the only thing I won't expect much of is specific interaction with/responses to other companions while traveling. Which I see as more of a bonus than a requirement in most games, so I wouldn't expect that as part of this stretch goal personally hehe. I guess we'll just have to see what their true scope is
*Edit: I swear I can math...