This plan looks ~40 years ahead, and takes into account a projected MEL population of between 6.5 and 8 million by that time, depending on who you listen to.
Not sure how anyone can definitively state MEL will not need an airport located in SE area at that time.
This is what planning is all about, and the (non) second airport in Sydney is a (not so) shining example of what a lack of foresight (not to mention political will and fortitude) leads to.
Given the population growth in MEL's SE in recent times an airport that side of town would seem to make perfect sense now, let alone in 40 years time.
As far as a rail link to Tulla goes, white elephant right now AFAIAC, but in 20, 30, 40 years? Who knows? (Certainly not me).