There's another micro-optimization in play here: tipically, writing here is understood as the process of turning a 1 to a 0 (flash cells actually reset to a "1"

There's another micro-optimization in play here: tipically, writing here is understood as the process of turning a 1 to a 0 (flash cells actually reset to a "1" value), and that transition is called "programming". If a write can be accomplished solely by programming, then it can be done in place, without having to reset the block or find an empty (filled with 1s) block to program the write to it.