I once remember Mr. Tory talking about how the Scarborough Town Centre was the only remaining "city centre" in Toronto without adequate transit (as a reason for building the subway), and I couldn't help but think about South Etobicoke.
The mayor probably used the name of the mall for the whole area. Scarborough's city centre is far more than just a mall: it has a civic centre, public square and an urban district of office buildings and condos. It's easier to justify building rapid transit to a district like that versus just a mall.
Mall owners sometimes promote their properties as "town centres", but only when convenient to commerce. They're not going to tolerate many of the kinds of activities that happen in real city centres: protests, youth hanging out or even something as simple as a person handing out brochures regarding their views on a contentious issue like abortion.
It would be good to extend the subway to Sherway Gardens because it's a major destination for consumers. It could be the centre of a dense city neighbourhood one day. It would also provide a new hub for suburban bus routes. To remove the many Mississauga Transit buses driving through Toronto's streets to Islington Station would be a good thing. Sherway Gardens is quite close to Mississauga.