Never heard that before. But it comes back to the same thing. There is no one reason why some write Mc while others write Mac. In my family we write it Mc with the c looking like a reverse apostrophe. If you did research at any of the Family History center, you will find the McKinnons are listed has M'Kinnon.
In the book Memoirs of Clan Fingon, you can see that our name has been through a lot of changes. One of the old spelling is M'Kynnoun. But we are all descendant of King Alpin son Gregor and his son Findanus. So at some point the Clan members where known has M'Findanus. Keep in mind that not many people knew how to write then. This could have contributed to our names many spellings. As for my branch, we have been spelling it McKinnon at least since 1825.
Most of my own knowledge of the Clan history comes from Memoirs of Clan Fingon by Rev. Donald D. MacKinnon, M.A. and Scottish Highlanders a personal view by Charles MacKinnon. Charles MacKinnon of Dunakin, is a hereditary chieftain and laird of Dunakin, and owner of Dunakin Castle. He is also 33rd in an unbroken male line of descent from Findanus, the fourth MacKinnon chief. He is also heriditary seanachaidh (genealogist and historian) of the clan MacKinnon. We are lucky to have so much information available to us. We have a long recorded history that is well worth reading. Hope you will have a chance to enjoy it, too.