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.
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.
Mniej and częściej are both comparatives, so you'd be basically saying "less more often".
Technically, "mniej często" would be grammatical, however, a language corpus search revealed that it's used about a thousand times less often than "rzadziej" and it also doesn't sound natural to me in this particular context.