The statement "I worked on X at BigCo Y" does not imply that the person has enough knowledge about X to start a startup with. The problems you have when starting a new project under the wings of an existing company (especially one so large as Meta) are very different than you get when starting a new startup, so the experience does not carry over to the new company as some people expect.
Presumably making your own Slack, or more generally spending time on things you don't have a competitive advantage at by reinventing someone else's wheel