For irreversible computation, yup. If reversible computation turns out to be practical, things are a little weirder, and the bounds are... further away.
The Landauer limit is dependent on temperature, though. If you fast forward the universe a little bit to let the CMB cool down, even irreversible computing can get pretty efficient in theory.
They don't have to. You can interpret the original comment as "puts an upper bound on the computational ability of a Dyson sphere if it dedicated all captured energy to computation".
