Check reverso.com and you will see nadamos is often translated as "we're swimming" Check any english text on spanish grammar and you will see the present can be translated as either.
If you want to ensure you are ONLY translated as "we are swimming", then yes, you are right - but "Nosotros nadamos" can be translated as 'are swimming' correctly, it's just a somewhat less frequent translation.
In Spanish, like in English, the present participle (words which end in "-ing"), is translated differently. To say it in Spanish, you need to use "estar", followed by "nadando". -AR verbs end in "-ando", while -ER and -IR verbs end in "-iendo". "We are swimming" would translate as "Estamos nadando".
I believe these forums are attached to this sentence, not any particular question form. So if jrslam was probably answering a translation question, and you were answering a listen-and-type question, both discussion links would lead to the same place - here - since it's the same sentence, just used in different question forms.
I'm no expert but I'm pretty sure that should have been wrong. Nosotros has nothing to do with past or present tense as far as I know. Most verbs have a completely different word for past tense (example: I ate is yo comi). If it gave you credit, I'm guessing it thought you typoed.