As another Swedish-as-mothertongue Finn, I'm 99 % sure that Linus speaks fluent Finnish.
There are Swedish-speaking people in Finland that don't speak Finnish particularly well, but they tend to live in the rural areas along the western coast or in the southwestern archipelago. Among the older population there, you might even find monoglots.
However, for someone grown up in and studied at uni in a city like Helsinki where 95 % of the population normally speaks Finnish, like Linus did, it's just inconceivable to not speak Finnish.