The tour may be 6.5 hours long but starting times from Native Choice could dictate when that clock starts. Could be others on the tour as well. Also, Most likely your actual time in port off the ship could be 8:30P (or later) -5:30P . So given traffic,etc. You'd probably need to be back at Native Choice by 4 PM or 4:30PM. If you left native Choice at 9:00 or 9:39 you'd be cutting it close. So that's probably why they say not enough time.
No clue about Toucan Tours.