They briefly (hah) covered it in TFA:

>Researchers have used cotton strips and specialty tea bags for similar purposes before, but those methods require precise scales and careful handling. A pair of underwear, by contrast, offers something immediate: anyone can look at what came out of the ground and grasp what it means.

The linked manuscript basically says the same:

>Cotton strips have previously been applied (Harrison et al., 1988; Nachimuthu et al., 2007), while the Tea Bag Index (TBI) makes use of commercially available teabags with standardized composition for the same purpose (Keuskamp et al., 2013). Due to the wide availability of the material, the TBI has been employed in a citizen science approach to gather global decomposition data (Sarneel et al., 2024). However, despite being relatively easy to apply and interpret, the method requires careful handling and specific tools, such as a fine scale balance. Moreover, it lacks visual appeal and may struggle to attract broader public attention for soil-related topics. A more playful approach that has occasionally been used to illustrate soil decomposition, though without scientific rigor, is the burial of cotton underpants (Figure 1).

It looks like establishing cotton underwear as a viable method was exactly the point of this study. As they say, "This work demonstrates that underwear decomposition varied widely across land-use types and that buried underwear is a simple and easily interpretable tool to gain a first impression of soil health."