Sorry, but this is plainly wrong. Please read all the details above (or here if even more details are needed).
It’s correct that the first boot turns off OEM unlocking. But you still can lock the bootloader successfully if by flasing you didn’t do a downgrade (of the patch level). What you can not do in this case is repair (unlock again) the phone in case you did a downgrade and the phone no longer boots.
(Still I’d not recommend to relock a bootloader with OEM unlocking disabled if you’re not 100% sure no downgrade has happened by flashing the OS!)