If he knows that you don't eat cholov stam then we assume that he won't give it to you to eat. (although part of the controversies with R Abadi was that he didn't do this. So I guess if Rav Abadi gave you a random piece of gum, are you allowed to eat it?)
As we see from the seifa, we know the reisha is not true. My hypothetical is that he's saying "it is 100% kosher" not that "it is Chalav Yisroel," because he maintains that it doesn't need to be CY to be "100% kosher" and I can eat it.
But even this is willful "deceipt."My main point, and the one under discussion is what is for more likely and common: an "eidus" that is based on ignorance of either halacha or food production. It's hard to imagine that R' Feinstein's psak is referring to such instances, even as a daas yachid.
yevamos 14a
תָּא שְׁמַע: אַף עַל פִּי שֶׁאֵלּוּ אוֹסְרִים וְאֵלּוּ מַתִּירִים — לֹא נִמְנְעוּ בֵּית שַׁמַּאי מִלִּישָּׂא נָשִׁים מִבֵּית הִלֵּל, וְלֹא בֵּית הִלֵּל מִבֵּית שַׁמַּאי. אִי אָמְרַתְּ בִּשְׁלָמָא לֹא עָשׂוּ, מִשּׁוּם הָכִי לֹא נִמְנְעוּ. אֶלָּא אִי אָמְרַתְּ עָשׂוּ, אַמַּאי לֹא נִמְנְעוּ?
etc
לָא, לְעוֹלָם עָשׂוּ, דְּמוֹדְעִי לְהוּ וּפָרְשִׁי.
Again, this is willful. The questions being discussed are predominantly not so