How do they plot the path of these things without knowing its weight and size? Seems like bullshit, especially when they specifically say "the sun will barely affect it" ?? The sun affects everything in proportion to the things you exactly dont yet know, doesn't it?

When you're dealing with objects as small as this, their weight and size is essentially mathematically irrelevant. It doesn't have any type of propulsion, so we know that unless it collides with something along its trajectory, it will keep following the same course. We know the mass and diameter of the celestial bodies that the object will be traveling near, so we can calculate how those bodies will affect its trajectory. As one of the top comments said: it has a very high eccentricity, which means it's traveling along a path that forms a very elongated, open hyperbole, so it won't come close enough to the sun to be affected by its gravitational pull if it continues to follow its current trajectory.