You're conflating Terminals and Gates.

In Las Vegas, JetBlue operates out of Terminal 3. When you arrive, your bags will be picked up at Terminal 3 baggage claim. When you leave, you will check-in at the regular JetBlue location and go through security at Terminal 3.

In the past, JetBlue used the E gates that are part of Terminal 3. Those E gates are closed. All flights now use D gates, which are located in a satellite building only accessible via underground tram. It will take extra time to get to your gate. It's been some time since I'd done that, but I'd guess it's about 15 to 20 minutes once you add elevators/escalators, the tram ride, and the walk to the gate.

Here's how McCarran explains it:
"Terminal 3 remains open with limited services during the closure of the E Concourse. Passengers will still be able to access airline ticketing and check-in, the TSA Checkpoint and Baggage Claim. Any flight previously scheduled to depart from the E Concourse will shift to the D Concourse. Signage is in place to direct travelers at Terminal 3 to use the Red Line tram to the D Concourse once they have cleared the security checkpoint."
Source (pdf): https://www.mccarran.com/pubfile/a87...0200603-093535

If you're a visual learner, here's a map:


Source: https://airportparkingguides.com/mcc...ional-airport/