I have not heard any Maadhwa saints talking about Shri-Madhavacharya writing commentary for Parashara smriti. Here is my level of understanding of that verse and actual meaning could be slightly different. God knows better!! I do not know anything!! :
In a village if there are many atheists and unclean people in the Treta Yuga, he should not stay in that village. Because the resources like water have to be shared in a village. Even if one is spiritual he will become contaminated by their presence in the village. In other Yugas the field (where the influence is there) of contamination varied. But in Kaliyuga there is no such contamination. The following line says that untouchability does not apply in Kaliyuga even if there are atheists and unclean people living in your colony or village or family (you should just avoid interacting with them (too much)):
in the Kali, one should shun the perpetrator alone (of an offence).)
Another verse on the same line from the Parashara smriti talks about field of untouchability in a different way:
” In the Krita sin is incurred by one who converses (with a sinner) ; in the Treta by one who touches (the sinful man) ; in the Dvapara by taking the sinner’s food ; in the Kali by a (sinful) act (alone).