Not quite. A FPL can be activated 15mins prior or after EOBT. If a pilot requests to depart prior to this it will have to be re filed for the earlier time. If later then a Delay message is sent.
If a CTOT is given, and the pilot reports ready before the slot tollerance (5 ins prior, 10 mins after CTOT), then a ready message may be sent.
If it looks like you will be out of slot tollerance, a call to flow, can 'sometimes' buy an extra couple of mins.