You are correct in that matter. You only gain EXP from submissions (Skins, Texturepacks, Mods, Ect.) Though posting them will give you a little bit of EXP, You need people to diamond, view, and download your submission to get more EXP.
Please note that this does not mean you should beg for diamonds as that is against the rules. If people really like your submission or if it is really high quality and amazing, you will level up in no time.
So really, just post good quality content, and eventually, you will become popular like some of the level 70 members out there. (I have not yet reached it.)
Sorry for the long post, but I hope it helped.