Tether is an unaudited black box, so no one really knows what their reserves are composed of. Tether's also been caught lying about their reserves numerous times [1], so it would be insane to think they're a credible store of value. Given their history and opaqueness, it's reasonable to assume shenanigans.

[1] https://www.dirtybubblemedia.com/p/examining-tethers-secret-...