Returning to my research on emoji reactions, there's a few points that I think are important to take from the affordance.
First is utility. Reactions are for cleaning up a chat and eliminating the need for short replies that convey very simple ideas. Replies like "lol" or "haha" or "what?" or "woah" or "nice" and so on (designers at Facebook identified the most common one-word responses in their process of designing reactions). In theory, all of these can be conveyed using an emoji (more on that in the next section). Attaching the emoji to the message also removes the need to quote, and aggregating emoji counts from all members of the group can make a message a kind of poll very easily. The one piece of functionality serves a number of very utilitarian purposes in quite a clean and efficient way.
The second idea is that reactions have, in the view of these companies, relatively agreed upon meanings. Since they have an effect on future message content, interpretation of reactions becomes essential. The 👍 should have a similar meaning to me as it does to the others who use it. Otherwise there's a mismatch between actual and perceived intent. Of course 👍 can mean different things in different contexts, but generally the intent should be well understood by others. If this isn't the case, then 👍 can't reliably be used as a replacement for "sure", "nice", "ok", "yes", and so on. It's a bit of a clash then; emojis can be as richly interperable as any other kind of language, but as a reaction they are designed to simplify and stand in for much more specific kinds of responses. For the most part that tension is resolved through the assumption that 👍 (and every other emoji used in reactions) is clear enough for the majority of cases.
Finally, there's something else that reactions provide, a kind of assurance that what you've said has been seen and interpreted. This is the non-verbal aspect to the affordance. It's a mark, a symbol that can be used to ensure you and others are on the same page, or to gauge how others are interpreting your messages. Or perhaps just that they've been seen. Reactions have another sort of utility in this regard.