***** NOT A TEACHER *****
(1) Congratulations on your excellent knowledge of grammar.
(2) It does seem that the BBC writer was distracted by those
words "it believes."
(3) If we deleted those words, then -- as you said -- we would
have "...on the momentum that has been created since ...."
(4) Of course, since a BBC writer wrote that, we cannot say
that it is "wrong." Maybe it is a great lesson for all learners and
us native speakers, too: sometimes the rules can be broken if there
is no real confusion. I have the feeling that most native speakers
would definitely have not noticed the missing relative pronoun.
P.S. It is not important, but you may wish to remember that here
in the United States, "cannot" is almost always spelled as one word.
Student or Learner