At a "team" size of 5000, the 10000-message limit means you each get 2 messages before things start disappearing. So I think the message limit pretty strongly implies a team member limit.

While their language was poorly chosen, I think what Slack meant was that their free tier was under no additional restrictions than their paid tier for team size, which seems reasonable.

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?

This is a completely legitimate point. They are expecting "team" to be self explanatory, and it is a vague term that is not self-explanatory. If there's a size limit, it would be best if they stated that plainly.

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".