I've only built a couple of sof kayaks but I remember this: the tenons are at an angle to the axis of the crossbeam so that they are at right angles to the gunwale. To maximize the width of the tenon for a given crossbeam width, it had to start at one side of the crossbeam and angle to the other side. The other side would essentially have a triangular notch cut from the end of the tenon back to the shoulder. Sorry, hard to describe in words. Thus it looks like the tenon was not centered, but will maximize the strength.

Will it be strong enough? Well, this design maximizes the size of the tenon for a given size of crossbeam, and given that I have read lots of references to using a couple of dowels (or even metal screws in the case of one maker) as being strong enough, I don't anticipate any problems.

Mark.