All true, but it also depneds wchich "ARRI" you are exactly referring to.
Alexa "Classic" (which basically boils down to everything but Amira and AlexaMini - MiniLF to be seen, as this is a weird hybrid of Mini and LF (A.K.A "purchasable-65", so "Classic"):
TC menu "Jam Sync" will, as Bouke said, "learn" TC and adjust internal reference - this requires a low jitter TTL TC signal and the better the signal the quicker the "learn" or "tune" (TC will blink while adjusting).
As this will also drive the internal reference it is as good as having genlock as long the camera stays on.
TC Menu set to "Regen" however, will regenerate the internal reference clock constantly clocked by incomming TC - so this literally is a "genlock by TC" Which means the signal must be constantly present like genlock, too. Again this may work unreliable with imperfect TC signals and put out errors.
Alexa "CAP2" cameras (Amira, Mini and possibly MiniLF):
TC Menu as above. Usually "Regen" does the job, due to the quality of circuitry.
But as the Amira was initially designed towards the ENG market it also was planned to accept separate external sync and they merged it by giving the additional option in sytem/sensor menu to reference clock the sensor as per Wandering Ear to TC in and even Genlock (still, (by default, the sync In BNC is not connected to any electronics, btw.). Selecting TC In on top hardlocks the whole system witht he same effect as on the "Classic" range.
Oh, and whoever really does need video sync: another contender of genlock spitting boxes was just announced at NAB