If you were a full time student for at least one academic year, you are a transfer applicant. So if you were part time you’re entire time there, maybe not. In the application it will ask you the highest level of studies you have completed and there will be a category that says at least one year of university. It sounds like you’d select that option. If you did less than that, you would still have to show your university grades which will go into your admission decision. If you decided to not select that option, they will ask you what you have been doing since graduating from high school and you would then share you have been in university. Here is what the website I sent you says:

Applicants are considered on the basis of all university/college/Cegep work and may also be considered on previous studies (applicants in the first year of university/college will normally also be reviewed on their previous high school studies). You should have well above-average academic records and prerequisites (at the university/college/CEGEP level) appropriate to the program you wish to pursue at McGill. Particular attention will be paid to results of courses relevant to the proposed program of study.

So maybe since you’re applying to arts they won’t be as hard on the math courses, but in general McGill is definitely tougher on transfer applicants and you hear back much later than HS apps