NAND technology - ecc error
No, it's all a game of probabilities. You just define a probability (e.g. \$1-10^{-14}\$) that all pages fulfill that criterion, the manufacturer models the stochastic process that leads to errors and makes observations of that process, and you so get to a probability that indeed your criterion is fulfilled.
There's different kind of errors. But yes, spontaneous bit flips can happen for thermal, particle / ionizing radiation, or current leakage reasons. Other errors are not spontaneous, but always immediately manifest. Note that a flipped bit doesn't mean the ECC must fail: Error checks are only safe against certain patterns of bit errors, and if multiple bits within a block (or page, or whatever you call it) fail, the ECC might not "notice" it. It's, again, a stochastic thing.
Only if


some recovery mechanism recovered the original data correctly; again, patterns, probability, and
the reason the page failed because a bit flipped randomly. Of course, if a bit cell is simply stuck to a value, then there's nothing rewriting could solve.