No, "retourner" is for some place you are not currently at (and you should specify where), and "revenir" is for where you currently are (so it is pointless to specify where you are).
I think you do not "retourner" to "ici". You can "revenir" to "ici", but you can only "retourner" to some other place, some place that you are not now at.
You may be correct.
But I should tell you that every French training program I have looked at insists that French requires a hint (at least) of a destination when talking about going to or from somewhere.
Typically, from about.com.....Je vais (I'm going) is not a complete sentence in French; if you don't follow the verb with a place, you have to say J'y vais. Duo is saying that retourner is a form of going and is subject to the same rule. That does not mean French speakers, in ordinary conversation, must necessarily slavishly follow this or any other procedural rule. On the other hand, it does mean that it is not rubbish to suggest that the requirement exists.
It is worth noting that a google search of Je retourne turns up countless pages with destination included. All kinds of destinations, many of them figurative but destinations or goals just the same.
Searching for English I return (with merchandise return policies, etc. excluded) turns up a much wider variety of possible uses of return. That includes simply ...I return...I could not find a simple Je retourne on the French pages. (except for conjugation purposes)
That indicates that some people think it is irregular to use je retourne without a marker for a destination.
If you mean the verb revenir has the destination implicit in the verb, that is you come back to where you started even if over a long period of time, then you are correct. The destination is known.
But of course you can return to anywhere you once were so retourner still requires that pesky y.
The point is that the destination is implicit, so there is no need for the "y". The "y", meaning "there", is included to resolve the implicit question "Where?" But if that location is already implicit in the verb itself, then there is no need to specify it with "y". At least, that's my theory.