Only you can evaluate what such a promise is worth. If the same managers have made promises to you in the past, or have made similar promises to other interns on the team, and have delivered on them, you might decide that there is a good chance that the thing being promised will come true (but it is still not guaranteed). One On the other hand, if there is no history of promises coming to fruition, there is no reason to believe that this one will.
This firm either has the money to pay you, and is choosing not to, or doesn't have the money. In either case, this is a bad sign. In the second case, where they are so cash strapped that they can't promote a talented intern to a jrJr. developer, this is such a warning sign about the stability of the firm that you should be heading for the doors. In the first case, it shows a firm that is exploiting your efforts, extracting value from you with nothing in return, and will continue to do so as long as possible. Again, you should be heading for the doorsexit.
First, you cannot compare yourself to others. There is always going to be someone who makes more money, has a better title, has an easier work load, etc., than you do. It is just the way it is. If this situation makes you upset, or lose motivation, you will need learn how to work on thismanage those feelings, because you will encounter this situation everywhere you go, over and over again.
Second, "you don't know what you don't know". As a brand new developer, how do you know that what the others are doing is "less demanding" or bringing "less value" thatthan what you are working on? Are you judging by hours spent, lines of code written, etc? because none of those are metrics of either the demanding nature of a job or the value broughtproduced.