Can you embed a main clause, or does it always have to be subordinate?

The definition of main clause is simple enough.

"A clause that can form a complete sentence standing alone, having a subject and a predicate." [Oxford languages]

"Main clauses make sense on their own. 'I like bananas. ' is a simple sentence which is made up of a main clause." threschoolrun.com.

There is a distinction between grammatical dependence and logical dependence.

Your example can be written as three grammatically independent sentences, joined by conjunctions in the form "A, but B and C". Each of the three can stand alone with its own independent truth conditions. You could write:

Some of you may not have heard of Dr Watkins. He is an expert in psychology from the University of Oxford. He said that “...”.

The use of but sets up a kind of logical relationship.

Although some of you may not have heard of Dr. Watkins , he is and expert....

All that (or something like it) can be done by that one short conjunction but. In the above version, I have grammatically subordinated sentence A to sentence B, which functions as the grammatical main clause. Arguably it, along with sentence C, is also the logically most important ('main' in the sense of 'most important' clause.

However, as quoted, your sentence consists of three clauses of equal standing.