The source you cited is for “People who are moderately or severely immunocompromised and initiate a Janssen COVID-19 Vaccine primary series”
It is sold under the brand name Jcovden and is a COVID‑19 vaccine that was developed by Janssen Vaccines in Leiden, Netherlands, and its Belgian parent company Janssen Pharmaceuticals. It’s also a viral vector vaccine (not MRNA), by the way. The source you cited says up to 3 doses (not 5) are recommended, depending on the dose history and type(s) of additional doses, two months after the initial dose (not every 3 months).
So, this “professional scientist” doesn’t know what he’s talking about.