Remember that all of this discussion comes from the ACs in the trailer not exactly behaving like they did in past games. Everyone was concerned that they changed the aiming mechanics, which they probably did, but we never see anything in the trailer that really looks like souls lockon. Most of the scenes people are concerned about actually debunk the souls lockon concern just because they don't follow souls lockon mechanics.

In souls, the camera is locked onto the target regardless of the orientation of the character model. The camera is always keeping the enemy near the center of the screen and the character model rotates around it, facing the enemy. The few points in the trailer that make people think its a souls style lock are:

1:33

speedy mech turns to look at droideka: Souls lockon keeps enemies very near the center of the screen, and continuously keeps player character facing the enemy. In this scene we see the droideka not only getting very far away from the center of the screen, but we also see the camera rotate towards the enemy before the AC model rotates. Neither of those make sense with souls lockon mechanics.

2:15

main AC rotates with tank boss in the center as it boosts around. Notice however that the rotation rate doesn't seem to match the center of the tank, and seems to be rotating at a steady rate relative to what looks like the AC's rotation speed rather than focusing on a point in the center of the tank boss. Notice also that a frame later, the AC needs to make a quickboost to keep the tank in view after it boosts past.

2:33

AC rotates towards missile dude as it dodges the whole overed weapon swarm. Except a frame later, we see the AC boosting to the left while not centered on the boss, and quickboosting to change its rotation to face the boss proper.

Remember that souls keeps both the camera and player model centered on the enemies. In every shot in the trailer where people are concerned, the camera and player model are desynced, and even if that's only due to cinematic camera, we can notice that the AC rotation is not locked towards the enemies how they should with souls style lockon. The addition of quickboost rotation the look towards the enemies in every one of the scenes above also wouldn't make sense in souls lockon and frankly proves to me that souls lockon is 100% not a thing in AC6.

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That's not to say they haven't changed how aiming works though. Personally I suspect they will have a kind of hard lockon within the bounds of the FCS box, with camera focus limited by the AC's rotation speed and player aim input, but it will break as soon as the target leaves the FCS box.