there are certain words that can act as either adverbs or connectors.
e.g.,
so
yet
then
for instance, both of these sentences are acceptable:
Traffic was slower than usual today, so I was late to work. ("so" = connector)
Traffic was slower than usual today, and so I was late to work. ("so" = adverb; this time the connector is "and")
'then' works this way, too.
in this sentence it's used without 'and'. it could also be used with 'and'; the author just didn't choose to write the sentence that way.