Acrylic tanks can be done that way. Intead of sealing the corners with silicone, you must use a welding agent. This chemichal "melts" the joint and then solidifies again. This way, if done properly, You'll only see a thin line where the acrylic planes meet. Maybe it was this way.
In the other hand, glass is rigid at room temperature, and it's very unlikely you'll get such perfect fit to avoid water from escaping. It could be done, but you'll need something else to hold the glass planes together, and precision equipment to give the exact size to the glass.
This would lead to a VERY EXPENSIVE and RISKY tank. For a glass tank with no visible joints, you'll need a curved glass tank.