Sounds from the article like it was used in factories, maybe to verify that a new production line or hardware revision is building consoles that function correctly?
In addition to what the others said regarding disassembly, it's a lot easier to distribute (and train remote technicians to use/maintain) a test cartridge to service centers around the world than to set up the $xx,000+ bespoke bed of nails test fixtures that you would use on the production line.
JTAG doesn't really exist on the N64 hardware, it's only present on the CPU and the pins are hardwired to VCC/gnd.
Even if it did work, it wouldn't be sufficient to test the rest of the system, which doesn't have any concept of JTAG.
The other major chip (RCP) does have an undocumented test mode, it allowed the verification guys at SGI introspection into various registers/state by exposing them onto the cartridge bus as gpio. I don't think this is written down anywhere though.
It's a production test suite, designed for use with a custom jig that plugs into all the various ports to fully test everything, not just the cartridge. The custom test jig named 'zaru' which if I recall is Japanese for a rice strainer (appropriate for go/no-go tests) can be detected by the test suite and unlocks all the tests. It even controls the operator pass/fail lights on the jig from this software.
Also, authorized Nintendo service centers got the carts as a way to diagnose customer issues, but obviously without the factory jig.
The funny part is that the stress tests parts actually render all sorts of interesting 3d models that engineers inside SGI grabbed at random. The Utah teapot, a rocking chair, a cow, a VW beetle, the SGI logo itself. All rendered into an off screen framebuffer and checksummed.
