You can expand the definition of "(enterprise) Productivity Apps" ad infinitum.
1. Slack from a very early point in their pivot away from games branded themselves as an Enterprise Productivity app and made comparisons to Enterprise Email tools.
2. Microsoft's inclusion of Skype for Business/Lync (and to another extent Outlook, especially given Slack's own email-competitive marketing) for years prior to Slack/Teams implies that Chat/Communications has a long history of being considered an Enterprise Productivity App.
Perhaps, but in the case of Teams you don't even have to stretch.