Rick Strassman's an academic and a public person. It is not easy to admit trying dmt because it's a controlled substance (questions of where he found it would arise from any interested parties) and he never really got permission to try dmt himself, only administer it to other people.

In some way, especially when it comes to psychedelics and how they are viewed by the mainstream scientific community, trying a strong, potentially life and thought-changing  psychedelic as dmt might have skewed his interpretation. It is far easier (just for the record) to not admit doing dmt because then you convince people that you maintain an fairly objective point of view with regard to the analysis of other people's experiences.

Admitting that he had also tried dmt would result in other people arguing that he's trying to pigeon-hole the subject's experiences to what he had experienced. You cannot easily and in most cases be both the experiment and the experimenter.