TD missing namespace but still passes the validations on JSON-LD and TD/OWL

Following TD from Fujitsu does not define the namespace {"iot": "http://iotschema.org/"} for iot:Temperature, iot:Humidity, etc, but it passes all validations in http://plugfest.thingweb.io/playground/

{
"@context": ["https://w3c.github.io/wot/w3c-wot-td-context.jsonld"],
"@type": "Thing",
"name": "Fujitsu-WiFiAgent240AC4114764",
"id": "urn:dev:wot:com:fujitsu:wifiagent",
"base" : "http://192.168.1.21/Things/Property/",
"properties": {
"Temperature": {
"@type": "iot:Temperature",
"type": "object",
"properties": {
"temperature":{"type":"number"},
"rssi":{"type":"number"}
},
"writable": false,
"observable": false,
"forms": [{
"href" : "temperature",
"mediaType": "application/json"
}]
},
"Humidity": {
"@type": "iot:Humidity",
"type": "object",
"properties": {
"humidity":{"type":"number"},
"rssi":{"type":"number"}
},
"writable": false,
"observable": false,
"forms": [{
"href" : "humidity",
"mediaType": "application/json"
}]
},
"AirPressure": {
"@type": "iot:AirPressure",
"type": "object",
"properties": {
"airPressure":{"type":"number"},
"rssi":{"type":"number"}
},
"writable": false,
"observable": false,
"forms": [{
"href" : "airPressure",
"mediaType": "application/json"
}]
},
"Dust": {
"@type": "iot:Dust",
"type": "object",
"properties": {
"dust":{"type":"number"},
"rssi":{"type":"number"}
},
"writable": false,
"observable": false,
"forms": [{
"href" : "dust",
"mediaType": "application/json"
}]
},
"AllSensorData": {
"@type":"iot:AllSensor",
"type": "object",
"properties": {
"temperature":{"type":"number"},
"humidity":{"type":"number"},
"airPressure":{"type":"number"},
"dust":{"type":"number"},
"rssi":{"type":"number"}
},
"writable": false,
"observable": false,
"forms": [{
"href" : "allSensorData",
"mediaType": "application/json"
}]
}
},
"actions":{},
"events":{}
}