The phase of development that is published isn't reflective of where things actually are; they get published when they are ready for public consumption/governance processes.
As mentioned above on funding, outline funding has been allocated via CRSTS rounds which makes it slightly more challenging for future Governments to reduce, albeit not impossible.
Let's watch this space..... (the space where the spades are supposed to go in 2028)....
As I've said, I will be amazed if ground-works are underway inside of 4 years, in fact I will be gobsmacked. More so because some of the first stages proposed by West Yorkshire Combined Authority, i.e. Leeds - Bradford may well require the use of existing heavy rail, which will then need to be planned for, wired, which is still a very long time off given that TRU won't be finished until the end of the decade. You have to also plan where the lines diverge, plan those works, go through all the public consultations and opposition, plan roadworks / closures / diversions whilst the work takes place including in city centres that are both currently being reshaped, etc, etc.
Let's face it, 2028 is a political pipe dream. Its just far enough away that most people won't question it, but not so far away that people will ignore it. Its a political sweet-spot between having to do something immediately and keeping it close enough to be able to use it in political campaigning. As the GE looms, expect lots of other projects to be promised for the second half of the next government's term.