“I asked one of the people who gave the shiur and he shrugged his shoulders and said “its not a yiddishe maichal”
This is what you wrote in the OP. Now, please go back and ask said anonymous Rabbonim what they mean by “not a yiddishe maichel”. Do they mean these are foods that didnt originate with yidden (in which case, pizza, hot dogs, tuna fish and cream cheese would fall into the same category, so why pick on sushi) or do they mean these are foods not associated with yidden (which is probably true of (80% of all items found in a heimishe supermarket again, why pick on sushi) or do they mean this is food yidden should not eat, in which case, we are asking you to ask said anonymous rabbonim why they believe this is the case. Of course there is the possibility that none of my suggested interpretations are correct, in which case, please ask them to clarify what they mean.