I find it kind of strange when use HTML to post (which some times mess up my test after sometime when it look perfectly OK when I posted), I cannot use the emoji. However, the replies allow Markdown. So, I can do the same here as you have as example in this post "".
Now after reading this post, I tried to look at submit post again and found seems possible but then you cannot use the Editor or the HTMLoption anymore. For example, when submit a new post, in the body section, click "Editor", then it change to "Markdown". Next, I click on "Markdown" and it seems to allow the emoji codes in to be showing nicely. Think I need to experiment more. May have to drop HTML editing and switch to Markdown totally.
In conclusion: I do agree steemit should have this allowed and not just the Markdown option only.