I'm left handed, and, pantomiming this with a pencil on my desk, I find it easy not only to position my hand at my right temple, but to also switch hands and perform the same action with ease. Therefore I find it difficult to say that a left-handed person would not be able to accomplish this.
What could be criticized in this statement is the definition of the term "readily". If readily means "without prior thought or preparation", then, yes, if a left-handed person, whether by intention or accident, shot themself in the temple, it seems much more likely to occur through the left temple than the right (although a suicide probably cannot be said to have occurred without prior thought or preparation). However, if "readily" is interpreted as "capable of", then no, a left-handed person is entirely capable of accidentally or intentionally shooting themselves in the right temple, regardless of their handedness. Note that both of these interpretations are based on likeliness rather than fact; there can be no 100% guarantees, considering that a person may have intentionally obscured evidence, or that unusual circumstances might have produced unusual results.
A competent forensic scientist would be able to provide a far greater series of analyses and interpretations, particularly with evidence. One thing I might assume bears investigating is the angle of entry; a gunshot wound on the opposite side of the head would seem more likely to point toward the back of the head than one from the handed side, because the hand cannot easily reach all the way across and then reverse itself to a transverse direction. If a left-handed person has an entry wound in the right temple that points toward the back of the head, this might indicate that the shot was made with the left hand.