To avoid confusion and since they never replied, I'll reply since I'm have the same issue and believe I know what they are talking about. Plus this might help others for a temporary solution until Apple figures out a fix. After updating to Sequoia last night, the issue started for me.
Prior to updating to Sequoia, in Sonoma or whichever prior iOS people had, we had our secondary and Third monitors positioned above the Macbook in the "arrange display" in display settings (what he's showing in that diagram). What he's pointing to isn't the seem, it's where the dock would appear if you hovered over that section with the cursor (it won't move to both, just which ever you are hovering over at the time). With the update to Sequoia, the dock no longer moves if you have the monitors arranged above the Macbook in "Arrange Display". That's the main issue.
The only solution I have found is to put the monitors side by side and then the dock will move between monitors, which works but not how my monitors are positioned. So that's where the frustration lies since it worked perfectly before but now awkward and not intuitive.
So it's definitely a bug that needs to be addressed.