Microsoft has only recently started to expanded their Skype for Business API (most of it is still labeled beta). Unless Instabird implements any of these interfaces it won't be able to use Skype for Business as it's server. Currently the only examples I've seen using the API are web based chat clients, none of which I've seen in production at all.
As far as I am aware there no 3rd party IM clients on the market that work with SFB. The Skype protocol is proprietary as well - which is probably the reason why there are no clients.