External drive APFS encryption secure? I just erased a newly purchased external USB disk using the Disk Utility program and set it to use the file system APFS Encrypted. I entered a strong password and the whole process took about a minute, then the device was supposedly encrypted and ready for use.

I noted that APFS encryption uses AES-128 which seems good, but the fact that the formatting only took a minute seems weird. Using encryption software like TrueCrypt, to safely prepare a new device takes hours, even days, just to fill the disk with random data before any valuable data is saved onto the device. For this reason, I am concerned that my APFS encrypted device is not secure enough. 

Are my concerns legitimate or have I misunderstood something here?
Did you click click Security Options, and use the slider to set how any times the data should be erased? 

Writing over the disk is to stop old files from being accessible. 

The time to encrypt a disk depends mostly on how many files need to be encrypted and it they old files should be overwritten. 
As it is a new drive there won't be a lot of files to convert.