Does anyone know the control disc used (http://www.microscopy-uk.org.uk/mag/imgsep16/m-disc-test3.jp...) was HTL or LTH?

As tested, it doesn't matter because the disc didn't even have a UV safe resin. The lifespan of the data area is only meaningful if the rest of the disc is intact.

Granted, archival discs aren't designed for full-sun exposure to start with, so in theory, the failed disc could have outlasted the other under real-world archival conditions, and this test wouldn't reveal that.