Instead of making the claim, how about laying that out in detail?
Perhaps with a logical rational fact based explanation we could get past the "old wives tales" you don't like.

Commonsense argument:

Let’s assume 4-inch wide tape on the seam and that the bias tape seam is ‘stronger’ than a 0-90 tape seam. Because the typical fiber orientation of the hull and deck glass is not on the bias, this means that the seam is stronger than the hull and deck. Therefore, if the boat is sufficiently loaded (stressed) the hull or deck would fail someplace outside of the seam, perhaps just 2-inches away from the center of the joint. Since the failure load would be the same as if the joint were only as strong as the rest of the boat, there is no valid reason to have the seam stronger than the rest of the boat. Therefore, the most weight efficient seam has the fibers closely aligned with the fibers in the hull and deck, which are typically not on the bias.

Mechanics of fiber composites argument:

The failure load of any given fiber composite is strongly dependent on the combination of loading direction and fiber orientation. Lab tests clearly show that if we load a 0-90 orientation composite along the 90-direction (across the width of the tape), the strength of the tape is essentially the strength of the fibers in the 90 direction, and the 0-direction fibers have little influence. Lab data also clearly show that if we load a same weight bias tape (+45-45) in the 90-direction, the strength is much less than the 0-90 tape. Therefore, having twice as many fibers crossing the load axis does not make the tape as strong as the 0-90 tape.

Lab results clearly tell us that if we want to join two panels edgewise, and then apply loading such that the panels are displaced perpendicular to the seam, the strongest seam would have fibers aligned perpendicular to the seam. But if we want to load the panels such that they tend to slide parallel to the seam (shear loading), the strongest seam would have fibers on the +45-45 bias. This is not because there are twice as many fibers crossing the joint, but because the tensile stresses that develop from this “shear” loading are resolved at 45 degrees to the seam.

Take the kayak to be a simple beam, loaded in the middle by the paddler, and supported by a smoothly distributed load that is a function of the local displacement. If the hull to deck seam is close to the neutral axis of bending, the seam will see the maximum shear loading possible. But in this case, because the shear component is small, it is easily carried by even the least efficient fiber orientation possible. So the argument for bias tape to add strength is again not supported.
I hope this helps.

Sam