I'm finding the "positive sum" language around coins really misleading.

It's literally the opposite. If the way users make money from their content is when other users buy it, it's literally zero sum, financially. Not everyone can be making money, unless external capital is being injected from a another source (which is losing money).

Is there not massive amounts of capital injected into social platforms now?

No reason there can’t be an ad model built on top where the ads come thru users vs the platform