I think I have your answer

a descendant of the Delisle Alberta McMillans match my family's DNA...

your information on John McMillan and Ann Cameron is correct. Ann was the daughter of Donald/ Daniel Cameron and Margaret Fraser of Cornwall, Stormont, Ontario. John and Ann moved to the Owen Sound area with most of their siblings in the 1840s

More of the Camerons and McMillans are in the tree "Cameron loyalists"

Best
Kim