Yes, change to lowercase 'false' but also you need to remove some of the other attributes that are 'read-only'.  A working example is pasted below using 'curl':

curl -k --user username:password -X POST https://192.168.36.202:8080/platform/1/quota/quotas -d \
'{
               "container": false,
               "enforced": true,
               "include_snapshots": false,
               "thresholds_include_overhead": false,
               "type": "directory",
               "path": "/ifs/data/nfs",
               "thresholds": {
                   "advisory": 524288000,
                   "hard": 51200000000
               }
}'

The resulting quota looks like this:

yfvm-7105-2# isi quota quotas view --type=directory --path=/ifs/data/nfs
                       Path: /ifs/data/nfs
                       Type: directory
                  Snapshots: No
Thresholds Include Overhead: No
                      Usage
                          Files : 3
                  With Overhead : 7.53M
                   W/O Overhead : 5.00M
                       Over: -
                   Enforced: Yes
                  Container: No
                     Linked: -
                 Thresholds
                 Hard Threshold : 47.68G
                  Hard Exceeded : No
             Hard Last Exceeded : 1969-12-31T19:00:00
             Advisory Threshold : 500.00M
              Advisory Exceeded : No
         Advisory Last Exceeded : 1969-12-31T19:00:00
                 Soft Threshold : -
                  Soft Exceeded : No
             Soft Last Exceeded : -
                     Soft Grace : -