it depends on your dialect. I think most Americans would see "more seldom" as perfectly natural and correct. It is even part of a phrase: "More seldom than not." But for what it's worth, I believe that one originated in a Bush speech…
I am concerned that the Duo team is adding alternatives that are not good English without proper vetting. And I personally do not consider a Google search for frequency as proper vetting. This "seldomer" issue is not the only example I have seen. Yes, I know we are not teaching English here, but there is no reason to add poor English to the list of correct choices just because someone suggests it. I gather that there are non-native speakers of English here learning Polish also, and I don't think we should be reinforcing poor English.
Who defines proper English? Proper English generally means whatever is correct in the dialect you belong to. The problem here is that English has dozens of dialects and the Duolingo team decided to accept more than just American Standard English (the goal of this program). There is nothing "poor" about many of these constructions in specific dialects. There is no easy solution because if these aren't accepted, millions of people will feel upset that British English and many forms of American English are not accepted. If I had to guess, I'd guess that you are British. Would you be upset if Duolingo rejected "colour," "whilst," and "realise" just because it is incorrect in American Standard English?
If you search for "seldomer" on Google, go past the dictionaries, a couple names, and some old Google Books hits, there's a quote of 19th century poetry and then there's this page. There seems to be no Internet usage of "seldomer", and it's hard to find a recent cite of it in Google Books. It's not good modern English of any dialect.
Here https://en.wiktionary.org/wiki/Wiktionary:Tea_room/2016/November#seldomer the same conclusion was reached, it seems.
I suggest you stop guessing. It's not working. I'm not British just because I used the word "proper".
My point is still valid and I tried to guess only to make it relevant to your position in the language.
For my tuppence worth:
I am running and I go running can mean the same thing in English. "I am running" does not necessarily mean continuously. It would be quite proper to say in English, "I am running everyday" without implying that I never stop.
That is perfectly fine English, and implies that the habit of running for exercise is continuous, not that the running itself never stops.
Interesting comments, I also had run less and your explanation makes your point valid. The problem is in the states, depending on where you live, everyone has their regional dialect and way of expressing them selves. I believe there are few languages that are as fluid as English and as such a certain criteria must must be met for stability. Of course that's only my opinion.