They usually are. It’s a process akin to archaeology where they have to carefully wash away the dirt from the root system, measuring as they go. The problem with this method is that it's hard to reconstruct the entire 3d structure of bigger plants like trees so a lot of the root drawings on the site don’t accurately show how deep they go. It’s much easier with small plants where the researcher can control the soil used.

Modern methods like xray CT or ground penetrating radar can do it nondestructively in the field but they’re usually expensive to set up compared to just sending some grad students to dig.

I had assumed they had grown the plant between two vertical, parallel panes of glass.