I'm leaving the Marine Corps's DEP but will they truly de-enlist me?

you are correct the Marines are going to try and convince you that because you signed a contract you have to report to Meps to ship out this is 100% not true . A second Oath is required to enter onto active duty Simply refuse this oath and your not going anywhere . tell the recruiter your not going , you have changed your mind and refuse to take the oath , there is NOTHING they can do to you