Why are they even considering Slack which is a proprietary IM service? Come on, they are Mozilla. Matrix which is federated and is proposed like a "better XMPP" is more like it.
Why should federation play into anything here? This is specifically something for people to talk to each other under the auspices of Mozilla's work efforts.
Non federated IMs are a major problem. So using them means proliferating this issue, instead of fixing it. It's not solved, because major players refuse to solve it even though they can. When Mozilla uses stuff like that, it gives them endorsement.