This is obvious. The question of whether or not one should shukl, is a well known dispute among the authorities. Some, such as the Sefer Hassidim hold it is a mitsvah that one must do, in a accordance with the verse "all my bones shall say, who is like you" while others, such as the Arukh HaShulhan and Mishnah Berurah hold that it is not necessary; rather, whatever is best for one's concentration should be done.
Ben Bukhri holds like the Arukh Hashulhan/Mishnah Berurah, while R. Yohanan holds like the Sefer Hassidim.
Ben Bukhri's point in stating that it is not a sin, is to disagree with the view of the Ba'al Shem Tov, et al. that shukling is inappropriate and disrespectful. Rather, asserts Ben Bukhri, it is permissible. (While R. Yohanan holds it is actually required).
That much is obvious, the question of why they speak about kohanim in particular is less clear. Perhaps Ben Bukhra's point is that even kohanim who are supposed to live on a higher level and act formal and proper as spiritual leaders, may nevertheless sway, an act which we have noted, many consider to be disrespectful.