As above

I was traveling from Boston to Toronto in mid November on Business Class (which I opt as a paid upgrade from Economy). As Air Canada does not operate its own lounge, I was told by the counter to go to United Club.

At the club, I was denied access as I was told that the club is only for onwards Business Class passengers on international flight (from YYZ) and as my flight ends in YYZ, I was not eligible for entry.

As I did not have much time, I decided against arguing.

Perhaps someone can help shed some light on this?

Was my access denied as I was traveling only on a 'domestic' J or perhaps upgraded tickets does come with partner lounge access?

Thank you