The use of temporary nodes as jigs or templates can help with alignment in yEd.
To center nodes within a vertical swimlane, create a temporary rectangular node that you have adjusted to match the width of the swimlane. Just add a simple rectangle to the swimlane, stretch it to touch both sides of the lane, and reduce the height if needed to avoid other nodes in the lane.
Select all nodes in the swimlane including the temporary rectangle. Align the nodes horizontally.
With all of the nodes still selected, drag the set of nodes left or right until the temporary node again touches both sides of the lane. Deselect all nodes. Delete the temporay node. The nodes are now centered in the swimlane.
This takes a few more steps than simply aligning horizontally, but quickly achieves the desired result.