The journal article actually touches on that:

>In CD participants, indoles such as indole-3-propionic acid (IPA) and indole-3-carboxyaldehyde (ICA) and the neurotransmitter γ-aminobutyric acid (GABA) were significantly decreased.

GABA is an anti-anxiety neurotransmitter, it's what benzodiazepines mimic.