Have you tried using blkdiscard instead of fstrim?
Are you logged into the vCenter as root?
VM Hardware version is 13 or higher?
Turns out the official 6.7U1 VCSA ISO deployed it as HW version 10 instead of 14. (HW Compatibility with ESXi 5.5 and later).
I couldn't find anything about the pros and cons on upgrading the VCSA HA compatibility version, so I'll leave it as version 10 for now.
On the other hand, I tested and found that the VCSA does not have the "blkdiscard" command, only "fstrim" - which, as mentioned, does not work.
So, even if I upgraded it to HW version 14, neither commands would work anyway.
Is there an official word on doing TRIM/UNMAP on the VCSA itself? Seems like a no go at this point.