Seems to be a minor typo . Paper:

>17.5% of the measured value for Terbium-Gallium-Garnet (TGG) at 800 nm, and up to 75% at 1.3 µm.

Here's what the crystal looks like

https://www.photonchinaa.com/tgg-terbium-gallium-garnet/

Here's transmission plot (UV-IR)

https://www.samaterials.com/terbium-gallium-garnet-crystal.h...

Note there's almost no effect on transmission

Relevant? https://dspace.mit.edu/handle/1721.1/51819