I asked Mr Gaiman on Twitter to respond, and he did, as well as another person:
Aliza "I think and I vote." (@alizatw):
To summarize: it's reported but not authoritatively confirmed that
@neilhimself and @terryandrob did a radio interview with someone who
thought they were religious zealots and #GoodOmens was talking about
the real Armageddon.
I haven't found a transcript or recording of the interview itself, but I did find a transcript of a different interview where the two spoke about this interview:
Neil Gaiman: "The first radio interview we did in New York, the interviewer was asking us 'Who is Agnes Nutter? What is her history? Is Armageddon happening?" and so on and so forth. ...
It is a reference to the song ‘San Francisco (Be Sure to Wear [Some] Flowers in Your Hair)’. Written by John Phillips and performed by Scott Mackenzie, it was one of the best-selling singles of the 1960s and has been called the unofficial anthem of the counter-culture movements of that decade.
If you're going to San Francisco
Be sure to wear some flowers in ...
Holmes and Watson are the criminals being chased by James Moriarty and Sebastian Moran.
Some of the clues about the main characters:
It mentions 'The Dynamics of an Asteroid' as the paper written by the detective friend; this is written by Moriarty in Doyle's works.
The narrator tells what a crack shot he was, as Moran is -- and as Watson never was