I guess that if you take the literal meaning of a word, you can call yourself anything you want. For instance, I could take some snaps on my smartphone, upload them to a stock photography site, make a few sales and then call myself a professional photographer. I’m a photographer because I took a photograph, and I’m professional because I made some money. I’m sure that you and most people in the photography industry would agree, that in real terms, that doesn’t make me a professional photographer by any stretch of the imagination.
For some really great information about the role of a music publisher, read this great article in the Sound On Sound magazine:
Having said all that, publishing is simply the process of making something available to the public, which is what happens here at AJ, but as @boomopera has said, in your scenario, the publishing is being done by Envato, since they are supplying the means and platform on which to sell your music. If you were to set up your own website and sell your music like bensound did, you would have a better claim to be able to call yourself a music publisher. Even then, I would be inclined to be more specific and call you a ‘stock music publisher’ since the term ‘music publisher’ is so well established within the industry to mean the role explained in that SOS article.