He mentioned he saw the contacts through the light through the microscope. I would guess it is impossible that he couldn't have noticed traces going perpendicular(?)
The tape is made of a pressure sensitive acrylic adhesive filled with conductive particles. I don't think you would see any traces in the tape itself, but the particles are spaced out enough to be insulating from left to right, but conduct only top and bottom. This is mentioned in the PDF I linked to in "Does DIY Beamer still exist?"
Here's a quote from the PDF:
"3M™ Electrically Conductive Adhesive Transfer Tape 9703 is a pressure sensitive adhesive (PSA) transfer tape with
anisotropic electrical conductivity. The PSA matrix is filled with conductive particles which allow interconnection between
substrates through the adhesive thickness (the “Z-axis”) but are spaced far enough apart for the product to be electrically
insulating in the plane of the adhesive. The PSA tack properties and lack of any thermal curing make tape 9703 easy to use
in assembly operations.
Tape 9703 electrically connects and mechanically bonds medium pitch flexible circuits with other flexible circuits (flex),
rigid printed circuit boards (PCB) or LCD screens. Electrically conductive tape 9703 offers good adhesion to common
PCB substrates such as copper, gold, FR-4 epoxy, Kapton™ polyimide and polyester films. Stable electrical performance
in any flexible circuit interconnection application may require mechanical reinforcement (clamping).
Tape 9703 also electrically connects and mechanically bonds EMI/RFI shield and gaskets to metal frames and enclosures.
The low contact resistance and tape construction result in good EMI performance. Tape 9703 can be applied as die cut
parts or in roll form and has good adhesion to common EMI/RFI substrates such as aluminum, stainless steel, and smooth
It mentions mechanical reinforcement may be required to ensure stable electrical performance in flexible circuits. Perhaps something on the join would've helped? I still stand by my first guess. The tape was used in the wrong axis, either that or the pitch was too small for the tape (it's supposed to work down to 0.4mm pitch)?