Highly excited states of the 0.79 eV luminescent defect observed in photoluminescence excitation (PLE) measurements are interpreted as effective-mass (EMT) states of a pseudo-donor with E_i=38.3 meV. The interpretation implies that the 1s ground state of the donor electron is fivefold split in the C_1h symmetric strain field of the defect. Modelling of the internal deformation around the defect by a compressive uniaxial field of 80 MPa along (001) allows the authors to explain the ground state splitting quantitatively and gives rise to a re-interpretation of published uniaxial stress data. Transient decay data suggest that the lowest excited defect state is a split singlet-triplet bound exciton, the 0.79 eV line being emitted by the higher-energy singlet.