In Dutch, like actually in most Indoeuropean languages, the continuous/progressive forms are optional if they even exist. Dutch has one, which in this case would be "Wij zijn aan het eten" (literally: "We are at the eating"). It's not rare, but I think in this case most people wouldn't use it because (1) the progressive is implied rather than something that needs stressing, and (2) it's a bit clumsy, especially if you also have to stick the light lunch in somewhere. (I think it would be "Wij zijn een lichte lunch aan het eten".)
In English it is not optional. Therefore it is often necessary to use it in English even though it's not used in Dutch.
Just to be completely clear: Interchangeable isn't quite the right word. In most European languages using the progressive is optional, but in general you can't use it to express actions for which English wouldn't use the progressive, either. The following are all equally wrong, except perhaps in a very weird (science fiction?) context:
I am being old. Ik ben aan het oud zijn. Ich bin am alt sein. Yo estoy estando/siendo viejo. Io sto essendo vecchio. Je suis en train d'être vieux.
The difference is that in English and in the Celtic languages, you must use the progressive to express I am eating right now, whereas in the other European languages you can say something like I eat right now instead.
In Italian you can unless you wanna mean something happening during the course of another action. In this particular example instead it would be preferable to use the -ing form, because the present means something that NORMALLY you do (having a light lunch), while I guess in this case the sentence simply describes what is happening at the moment, without any hints on the singularity or less of the action. Hope it's clear enough
The trick is to know the rule for the -e and add it in writing where appropriate.
The -e is the last thing that remains from a complicated conjugation system that still exists in German in almost full form and is already completely gone in English. Clearly it is currently disappearing in Dutch, too. In practice this probably means that some speakers don't speak it at all and have trouble remembering when to put it in writing, and many think they speak it but don't necessarily do it, at least not in a way that can be observed by others. Except when they speak very carefully for some reason.
I wouldn't worry too much about it. We could all decide to use an alternative, more progressive form of Dutch without any of these conjugation -es. The only problems would be getting Duolingo answers counted as wrong and not conforming to the rules of Dutch orthography.