Menachem, the word hilula is used by large swaths of klal yisroel to refer to the yahrtzeit of any tzadik, based on the lashon of the zohar; it’s not only in chabad.
I agree that the Lubavitcher rebbe never said that lag baomer is not the yahrtzeit of rebbe shimon; some simply enjoy stating contrarian opinions nonchalantly, and then reveling in the resultant fallout.