I believe the idea is that if the encryption key is protected with only 4-digits, you could brute-force it offline (if you cracked open the phone and de-soldered stuff). If the encryption key is protected with a secure passphrase (as, for example, PGP private keys typically are) then that attack becomes a lot less feasible.
Yeah, the online attack defence like a short password is sufficient to defeat most attacks so long as root is not enabled, and Google/Apple don't comply to remotely unlock the device or reset the password (or you have all google framework apk's ripped out, or not built). The phone should reboot or wipe itself, or timeout or do something besides allowing unlimited attempts.
The offline attack you need a password suitable for protecting against police GPU cloud running john the ripper. Android you can set this up (2 different passwords), but should then make a script that deletes adb and su, add it to rc.local and reboot. Also helps to sabotage the recovery partition so it deletes user data should anybody try to flash something to system image
There's also mobiflauge, which is experimental deniable encryption and has 2 passwords, one to open a decoy install and one for your secret files full of stolen government intel you took pictures of to fool casual searches, and not ripped apart JTAG forensics.
