No it would NOT require a brand new terminal.
I guess we will have to wait and see then. If Luton intends to handle more than 18 million passengers then they need to publish their intentions now as this existing upgrade was first proposed in January 2012 and will take 8 years to complete from first proposal and there wasn't a public inquiry.
BBC News - London Luton Airport hopes to boost passenger numbers
I can think of many issues regarding expanding the present terminal and the CTA that can't be expanded that I won't bother listing here but every issue requires more space something that is sadly lacking here at Luton.