From the press release.
Inadequate rail links are significantly limiting the airport’s ability to provide the air capacity that UK travellers want and that the economy needs. LLA is London’s fastest-growing major airport but is the only one without a direct, express-style rail service. The airport is calling for the introduction of four fast trains per hour at Luton Airport Parkway station as part of the new East Midlands rail franchise.
A lack of 4 non stop trains an hour won't stop the airport reaching 18 million although it would be nice just like more seating in departures which the airport does have control over. Also it seems that Luton has forgotten about Southend in its press statement.