Arakin 4: Ambiguous antecedent - to whom did Rabban Yochanan ben Zakkai speak?

In today's daf yomi, Arakhin 4, we see the following conversation:

דתנן אמר רבי יהודה העיד בן בוכרי ביבנה כל כהן ששוקל אינו חוטא

 As we learned in a mishna ( Shekalim 1: 4) that Rabbi Yehuda said that ben Bukhri testified before the Sages in Yavne: Any priest who contributes the half- shekel for communal offerings is not considered a sinner, despite the fact that he is not obligated to contribute.

אמר לו רבן יוחנן בן זכאי לא כן אלא כל כהן שאינו שוקל חוטא אלא שהכהנים דורשין מקרא זה לעצמן וכל מנחת כהן כליל תהיה לא תאכל

Rabbi Yehuda added that Rabban Yoḥanan ben Zakkai said to ben Bukhri: That is not the case; rather, any priest who does not contribute his half- shekel is considered a sinner, as they are obligated in this mitzva like all other Jews. But with regard to the priests who do not contribute the half-shekel, in order to excuse themselves from the mitzva they interpret this verse to their ownadvantage:“ And every meal offering of the priest shall be wholly made to smoke; it shall not be eaten” (Leviticus 6:16).

Who is לו in the phrase אמר לו רבן יוחנן בן זכאי? The two possibilities are:

a) Rabbi Yehuda
b) Ben Buchri

Koren, and therefore Sefaria, get this right. The English translation makes it clear that this second statement is a continued speech by Rabbi Yehuda, and that Rabban Yochanan ben Zakkai's reaction to is ben Buchri himself.

Artscroll, meanwhile, has it as "Rabban Yoḥanan ben Zakkai said to [Rabbi Yehuda]." This seems unlikely.