The whole world is not a team. You can only say that if you're making an intentionally, consciously hyperbolic statement.
I am not interested in long and un-ending pedantic arguments about the extreme corner-case definitions of words. I maintain that an online community of 10k people does not fit even the most charitable meaning of a working "team". If that doesn't work for you, then you're only setting yourself up for surprise.
This argument that "any size" should really mean "any size" amounts to someone walking into an "all-you-can-eat" buffet and expecting to be able to sit at the same table for 2 weeks eating all meals for the price of one, and then complaining that the 'all you can eat' advertisement should overrule all other policies the restaurant has.
It would be the same as someone seeing an ad for mini-golfing for $20 for a family of any size, and then bringing four thousand of your 2nd, 3rd, and 4th cousins, and demanding that the entire group gets in for $20. While there's a pedantic, technical truth to cousins belonging to your 'family', it doesn't meet anybody's common understanding of what an ad for family prices means, even the person trying to cheat the system.
If the author truly didn't even suspect that his 10k users is a massive stretch of the idea of a "team", the message limits should have been a clue, but instead of acknowledging he was pushing the limits, he blamed Slack.