Snapchat runs on App Engine (and other stuff runs on App Engine too) so App Engine is almost certainly bigger than Slack just based on that.
That seems... Questionable.
Question on how they measure - I'm in 7 slack workspaces. Do I count 7 times?
Well, I have to create a new account every time I join a new group, so I'd say yes.
Two if my workspaces are tied to the same enterprise account, they should be the same, but who knows how they measure. My base assumption is "in a way giving nicer numbers"