"Indieweb uses Slack" in the sense that, yes, there is an Indieweb Slack team. There are probably also people who contribute to the Indieweb who use Twitter and Facebook and other fully proprietary systems, as well as many who use Micro.blog, which is built using established open standards but does not make its server open source.
Don't be the guy who strokes his chin and says "You say you're a vegan, yet you have a leather wallet. Interesting."
Don't be the guy who strokes his chin and says "You say you're a vegan, yet you have a leather wallet. Interesting."