That's just false and Poles, Belarusians and Ukrainians get minor Ashkenazi regularly because of Pale of settlement and admix of Christianised Jews (obviously in small number hence admixture is in trace amounts) into society.
I saw tons of South Slavic 23andme results and almost nobody gets Jewish while northern Slavs do.
23andme has it refined to such level it can detect even below 1%. And if Mortimer gets that, a community and Ancestry shows the same than it is no mistake. We also have one part Lithuanian member here who scores 1/8 or 1/16 Jewish (I forgot) and he pretends that is "false" because his muh peper trail doesn't show it.

