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 sp3 orbitals get a little more p character, and the internal H-B-H bond angles decrease by about 12°.
Thus, the outer sp3 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 sp2 bond angle is just a coincidence.
Molecular Orbital theory gives a better explanation of the structure of B2H6, as in the video below.
