Saying that smilies are the same (or subset of) as emoji, because both can be used to expresse emotional mode by language neutral graphics, is like saying that french is the same as english because I can use both to buy bread and they use similar character set.
Written language is more then just representation of real and virtual concepts on paper/screen, it also serves to indicate social context. When I am writing in hebrew I can make some assumption about my readers that I can not make when using english.
Smilies are part of the “geek speak”. Geek speak roots are based at the era where communication was slow and text based, and it compromises the quality of the prose in exchange of brevity.
Emojis on the other hand are the language of girls and marketers and it is used when the language skills f the person are not developed enough to write a proper prose, when there is not enough place for proper prose, or when text is just too tuned down for your needs and you want to attract attention.
Both have their valid uses. My niece is more likely to understand the smiling emojy then a “:)” so I will send her an emoji and not a smiley, but if I will use emoji in technical context I will probably be laughed at.
Most software vendors understand that there is a difference, and let you use emoji and smilies even in the same context, except for wordpress which had decided they are the same in version 4.2. What a fail.