cellular chips not that easy to develop ey?
As I have said before, this is a foolish attitude.
Apple is certainly generating a large number of recent cellular patents, many of which are about ways of saving energy. (1% here, .5% there; each seem small, but they add up...) This makes sense insofar as radio energy is a substantial factor in the battery life of iPhones (and presumably the same tech and ideas will be used by Apple in subsequent BT and WiFi chips).
So one issue is that they're doing more than just "take Intel modem tech and move it onto an A-series chip".
A second issue is: how exactly do you want to implement the modem chip(s)?
One option is to place it on the A-series chip. That's "easy" but sub-optimal, because analog tech is not shrinking as rapidly as logic; you are paying a lot for N3 square millimeters that don't need to be in N3.
Alternatively you can have a separate chip placed on the PCB, like Apple does today. Problem is that uses depressingly large amounts of energy to communicate with the main A-series chip via PCB.
Optimal (probably) is to have the modem on a separate chiplet that's packaged with the A-series chip. But this involves a whole new set of design ideas and options. Do we place the chiplet on top of the A-series chip (like is done with A-series DRAM) or on the side (like is done with M-series DRAM)? How do we define the communications protocol between the two – use an existing (but sub-optimal) protocol or invent something new that can be used for future Apple designs (there will be more chiplets...) How do we split functionality between the modem chiplet and the main chip (for example should we have cache and a small CPU on the chiplet, or should most of the logic live on the A-series chip, and just analog circuits on the modem chip?)
Point is: Apple is defining here not just "let's move the Intel modem onto A-series". They are defining their RF-architecture for everything they make for the next five to ten years. Decisions made at this point will be what governs how everything from AirPods to Apple Watch to Mac Studio interact with their RF chips...
Better to get it right (and slip a year) than get it wrong!