Slack does quite poorly on accessibility, so anyone using assistive measures from their operating system or separate application (voice instructed navigation, screen readers etc) will have a much harder than the average blog post or mailing list.
Based on the context, it's pretty clear to me that the parent comment was not referring to accessibility in the a11y sense, but more of the "Slack is a private company and therefore inherently closed and evil" slant.
I personally find Slack to be utterly abysmal at actually keeping information, stuff that scrolls up beyond the fold is effectively lost. Maybe that's what GP meant?
