It's still a 150MB web-browser + attached web app, right?
Well yeah, it's not like how it's packaged changed the underlying design.
If you want a lightweight version of the slack client and don't fell like using the IRC or XMPP bridges you can use Epiphany (or Chromium) to install the website an an 'App'.
There is also wee-slack[0], a plug-in for weechat specifically that does more than the IRC gateway