La información de la Utilidad de Discos muestra que el disco encriptado no está encriptado.

El comando adecuado para comprobar si un dmg está cifrado o no es:

hdiutil isencrypted /../../encrypted.dmg

o más detallada con el tipo de encriptación:

hdiutil imageinfo /../../encrypted.dmg

Ejemplos:

hdiutil isencrypted /Users/user/Desktop/Untitled.dmg
encrypted: YES
blocksize: 512
uuid: 24895029-49B1-469D-A154-B8344B7CD6EB
private-key-count: 0
passphrase-count: 1
max-key-count: 1
version: 2

y/o

hdiutil imageinfo /Users/user/Desktop/Untitled.dmg
<Enter disk's password>
Backing Store Information:
    URL: file:///Users/user/Desktop/Untitled.dmg
    Name: Untitled.dmg
    Class Name: CEncryptedEncoding
    Encryption: AES-256
    Backing Store Information:
        URL: file:///Users/user/Desktop/Untitled.dmg
        Name: Untitled.dmg
        Class Name: CBSDBackingStore
Class Name: CRawDiskImage
Checksum Type: none
Size Information:
    Total Bytes: 100020736
    Compressed Ratio: 1
    Sector Count: 195353
    Total Non-Empty Bytes: 100020736
    Compressed Bytes: 100020736
    Total Empty Bytes: 0
Format: UDRW
Format Description: raw read/write
Checksum Value: 
Properties:
    Encrypted: true
    Kernel Compatible: false
    Checksummed: false
    Software License Agreement: false
    Partitioned: false
    Compressed: no
Segments:
    0: /Users/user/Desktop/Untitled.dmg
partitions:
    partition-scheme: GUID
    block-size: 512
    partitions:
        0:
            partition-name: Protective Master Boot Record
            partition-start: 0
            partition-synthesized: true
            partition-length: 1
            partition-hint: MBR
        1:
            partition-name: GPT Header
            partition-start: 1
            partition-synthesized: true
            partition-length: 1
            partition-hint: Primary GPT Header
        2:
            partition-name: GPT Partition Data
            partition-start: 2
            partition-synthesized: true
            partition-length: 32
            partition-hint: Primary GPT Table
        3:
            partition-name: 
            partition-start: 34
            partition-synthesized: true
            partition-length: 6
            partition-hint: Apple_Free
        4:
            partition-UUID: 17698BE2-A406-43AA-9195-8D1F1AAECE6B
            partition-name: disk image
            partition-hint-UUID: 48465300-0000-11AA-AA11-00306543ECAC
            partition-start: 40
            partition-number: 1
            partition-length: 195280
            partition-hint: Apple_HFS
            partition-filesystems:
                HFS+: 
        5:
            partition-name: GPT Partition Data
            partition-start: 195320
            partition-synthesized: true
            partition-length: 32
            partition-hint: Backup GPT Table
        6:
            partition-name: GPT Header
            partition-start: 195352
            partition-synthesized: true
            partition-length: 1
            partition-hint: Backup GPT Header
    burnable: false
Resize limits (per hdiutil resize -limits):
 min     cur     max 
36072   195280  60572704

Sólo puedo adivinar por qué diskutil no muestra que la imagen está encriptada. Es un fallo o diskutil sólo valora FileVault como cifrado de disco.