They did hard code a limit, the API has a specific error for that. They just choose not to disclose it, and to advertise with the phrase "Slack is free to use for as long as you want for teams of all sizes."
And signing people up with an undocumented API that skips a big amount of signup friction seems like a recipe for running into undocumented limits. Would we be upset with Slack for not supporting 1 million user teams? 1 billion? 1 trillion?
But, come on, you have to admit that an online community of thousands of people does not fit even the most charitable interpretation of the word "team".
That is basically what the whole thread and article evolves about. Are 10k people a team? Really? Sure, you can argue all day that technically it may be the case, the whole world could be my team, some kind of super team and so a service shouldn't make any assumptions and so on, but reality is that everyone who hears "team" has a pretty good idea of a ball park figure how many people that will be and that freecodecamp doesn't fit into this. So, in the end it's just playing around semantics "but, but, but ... you said of any size .."
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.