Need help understanding 9-closing-accounts secure vs insecure-still-still

[billythedummy]

Sorry I don't quite see what the purpose of the force_defund() instruction is or how it makes it secure in contrast to insecure-still-still. From its naming it seems like it's supposed to force the defunding of closed accounts? If so, should it be

if discriminator != CLOSED_ACCOUNT_DISCRIMINATOR

on https://github.com/project-serum/sealevel-attacks/blob/2902c7976a36ddd34e16022026e8d64cf1667974/programs/9-closing-accounts/secure/src/lib.rs#L41-L43 instead?

Is the fact that it's a different instruction supposed to demonstrate that the only way to force the defunding of a closed account is to start a new transaction that calls force_defund() after close() has been called in a previous transaction (because otherwise users are free to append TransferLamports instructions to the same transaction in which close() was called)?

[billythedummy]

saw coral-xyz/anchor#613, understood