The lexicon constructor example is explained to be equivalent to some code with bad syntax.
set mylexicon to lexicon("key1", "value1", "key2", "value2").
is said to be equivalent to:
set mylexicon to lexicon().
mylexicon["key1"] = "value1".
mylexicon["key2"] = "value2".
Should have been:
set mylexicon to lexicon().
set mylexicon["key1"] to "value1".
set mylexicon["key2"] to "value2".