Basically, it's common to use 'sein' with some intransitive movement verbs like 'schwimmen', 'fahren', 'fliegen', etc. even if there isn't a direction.
The question is in the present perfect (perfekt) tense. Er ist geschwommen = he has swum. Er schwamm = he swam (in the imperfect tense.)
German has been my second language since I was a little guy, but english is my first and no one says "he swum in the lake" people say "he swam in the lake" or "he went swimming in the lake". "Er schwamm" is called Präteritum or Simple past. "Er ist geschommen" is called the "Perfect" tense and in german is called "Perfekt"
Correct, the answer isn't "he swum in the lake" it's "he has swum in the lake". Without 'has' it would be, as you said, "he swam" because then it wouldn't be the present perfect, it would be the simple past.
Some native English speakers seem to think that "he has swam" is fine. It's not and it sounds horrible. The past participle of "swim" is "swum".
Anyone telling you that "he has swam" is acceptable is also likely to say things like "I have ran..." instead of "I have run..." and that's because they haven't studied English in any detail.
It's important that we all understand the difference between 'speaking colloquially' and 'speaking in an uneducated manner'. That's not meant to insult but learners have the right to know the difference. If you use the simple past with "have" and "had" instead of the correct past participle, you will sound uneducated. It's like saying "I would of" instead of "I would have". It has nothing to do with 'common usage' and is only common among certain 'classes' (I hate that word in this context but England has a class system and it's the only way I know how to adequately explain this).
In the dialect where I live ( which has nothing to do with being more or less of a language) most would never say "I have drunk so many glasses of water" , "She has swum in the lake for the last time." or " I have shaven for the last time" We removed that out of our speech, perhaps because it feels awkward to say and it just doesn't feel right to the ear. It is "proper" but not to everyone or most.
In Austria and Switzerland they also do not use certain grammatical rules in a similar way. They, as well as the Swiss, do not use simple past or Präteritum. But it is still a language, just a different dialect of one.
Yes, you removed "She has swum..." but I don't think you're saying that you replaced it with "She has swam"? Perhaps you're saying that the simple past is more common: "She swam"? In which case, that makes sense because we also do that here! Saying "she has swum three times today" is a bit formal so most would say "She swam three times today". That doesn't make the word "swum" itself archaic ('archaic' means something completely different!) because it's still the past participle of "swim", it just means that people don't usually use it because they use a different tense entirely which doesn't require the use of "swum". I believe I said this in my last post.
But to correctly translate from german to english you do not have to keep the hat/has tense structure and in many cases to do so would be a mistranslation. From what I have read there is no semantic difference between german preterite and perfect - they both mean exactly the same thing. This is not the case in english.
"He swam in the sea" means a different thing to "he has swum in the sea" and I think the german "Er ist im Meer geschwommen" means more the first.
"Swum" for english simple past is archaic and so really shouldn't be given in statement without "has/have".
Perhaps because you've never had cause to use the past participle when discussing swimming? I.e. maybe you've always used the simple past and said "I swam..."
If you use the past participle correctly in other cases, then there should be no reason not to for "swim" e.g. "I have run 1000 miles", "I had just begun when...", "I had seen the light" etc. (you wouldn't say "I had saw the light" would you?!)