Found the issue: The drive was marked as not local. Also it may be that the drive is not flagged as an SSD (not in my case though).
~ # esxcli storage core device list -d naa.600508b1001c4fb788c917f46bf2a2f5
Display Name: HP Serial Attached SCSI Disk (naa.600508b1001c4fb788c917f46bf2a2f5)
Has Settable Display Name: true
Size: 915683
Device Type: Direct-Access
Multipath Plugin: NMP
Devfs Path: /vmfs/devices/disks/naa.600508b1001c4fb788c917f46bf2a2f5
Vendor: HP
Model: LOGICAL VOLUME
Revision: 4.68
SCSI Level: 5
Is Pseudo: false
Status: degraded
Is RDM Capable: true
Is Local: false
Is Removable: false
Is SSD: true
Is Offline: false
Is Perennially Reserved: false
Queue Full Sample Size: 0
Queue Full Threshold: 0
Thin Provisioning Status: unknown
Attached Filters:
VAAI Status: unknown
Other UIDs: vml.0200010000600508b1001c4fb788c917f46bf2a2f54c4f47494341
Is Local SAS Device: false
Is Boot USB Device: false
No of outstanding IOs with competing worlds: 32
In order to mark it as local I ran this command (note that the syntax of option sub-command is wrong in the documentation, I posted the correct one):
esxcli storage nmp satp rule add -s VMW_SATP_LOCAL --device naa.600508b1001c277a2d4d5e0285b4f6db --option="enable_local"
After a reboot it was possible to add the drive as flash device.
Thanks