That is easy to check if you measure the amount of light reaching the eyes of the patient while the experience is happening. Without even checking, I am already quite confident that no extra light will be reaching their eyes because they took some drug, but it's easy to measure.
You are constantly being bombarded with sensory phenomena that your nerves detect but your brain ignores. For example, you smell almost nothing, nearly all the time, despite being able to smell those scents occasionally, such as when you move to a different environment. Changing your brain somehow to notice those phenomena would not change the physical phenomena.
The claim was that some external entity was communicating with the people taking DMT. However, others in the room did not detect that entity, so it can't be made up of normal matter, or at least not at normal sizes. It is possible that the entity detected the person taking DMT somehow and started contacting them from far away, but then an instrument could detect the change after the DMT is consumed.
The alternative is that the entity is communicating in some way that is neither electromagnetic nor gravitational nor the weak or strong interactions, which would require new physics, and it would also require some explanation of why our brains would have evolved to capture this fifth force of nature that somehow doesn't have any measurable effects outside of DMT.