Some authors have stated that only about 10% of Polish Jews during the interwar period could be considered “assimilated” while more than 80% could be readily recognized as Jewish
Being non-religious is not the same as assimilationist. the Bund were non-relgious and probably anti-relgious, but they were not assimaltionist.
The non-relgious jews in poland Spoke Yiddish, married among themselves , but also engaged in Jewish culture like jewish literature, Jewish theater, jewish Orchestras and other jewish cultural things.
An Assimated jew would be someone likely married to a non-jew and not engaging in anything jewish except maybe a last name (if even that)