Basically, it costs free energy to forget a bit. If you flip a bit, that doesn't mean you forget it, as long as you remember you'll have to flip it back (or reinterpret it).
("Basically" because there are whole books on the subtleties of physics and information. I haven't mastered them.)
This is the correct answer. If you don't overwrite a bit, but instead move the old value to an auxiliary bit known to be zero, that can be theoretically done for free.
From what I understand flipping a bit isn't necessarily subject to this bound. However setting it to either 1 or 0 is.
Of course if you can flip it without expending energy then reading it and flipping it if it's 1 necessarily requires the Landauer limit's worth of energy.
Well for example, T cannot be less than the temperature of the cosmic microwave background. You can make regions with lower T, but only by pumping heat out in some way, which is more energy to do.
Being on mobile and not able to explore in depth, that quote sounds like a variant of Maxwell’s demon. It is correct to say that the Landauer limit is not due to a single physical law that must hold true, but rather a lack of knowledge about the state of the universe and the fact that acquiring that knowledge to do a “free” bitflip requires at least equivalent energy expenditure as that bitflip. TANSTAAFL.
How does that square with Landauer's limit?
[1] Apologies for the lack of reference. I'll try and find it.