But it is a distorted tetrahedron. The bridging #"H-B-H"# bond angles are 97°, but the outer #"H-B-H"# bond angles are 120 °.
The bridging bonds are banana bonds.
To understand the bond angles, recall the discussion of banana bonds in the structure of cyclopropane.
The four bonds between the two #"B"# atoms and the two bridging #"H"# atoms "want" to get close to a square, so the internal bond angle will settle on some value between 90° and 109.5°.
The inner #sp^3# orbitals get a little more #p# character, and the internal #"H-B-H"# bond angles decrease by about 12°.
Thus, the outer #sp^3# orbitals get a little more #s# character, and the outer #"H-B-H"# bond angles increase by about 11° to 120°.
That the angle is the same as the #sp^2# bond angle is just a coincidence.
Molecular Orbital theory gives a better explanation of the structure of #"B"_2"H"_6#, as in the video below.
