Really surprised no one has posted the official rules
https://www.transportation.gov/indiv.../tarmac-delays
If the door was open of course this doesn't apply because the passengers have the opportunity to get off the plane and should exercise it if wanted. If the FAs are refusing to let pax off then I would say that there is "no opportunity" to get off the plane and the Tarmac delay rules apply. You can get off after 3 hours, but the airlines is not required to rebook you.