The other bit that people aren't mentioning is that with modem development in-house, Apple gets to set the timetable - when contracting with Qualcomm or Intel for modems, Apple could say "we really need this new version by mid-May" and the best they can hope is the other party says, "we'll take that under advisement / we'll probably do that". With the development in-house, Apple can decide it isn't going fast enough to hit a particular deadline, and throw more resources/money at it if they deem necessary. They're not stuck with someone else's timetable (probably another reason they'd like - eventually - to switch some Macs from Intel x86 flavor chips to in-house ARM chips).
And, the new modem design, rather that catering to the needs of "Apple and others", only needs to cater to the needs of Apple; capabilities/features Apple doesn't need can be dropped from the die - and the timetable. Every time an Apple hardware engineer said/says, "but it would be really useful to us if the next chip had capability X, or would implement Y in Z way", now they can send that request on up the line, and the top management can decide to say, "make it so." It might give them more of a seat at the table for shaping future wireless standards, too.
Also, rather than having the other party say merely, "work on the new modem we spec'd is progressing nicely (or not)", or "we're dropping spec'd feature X (that you wanted) because we decided it's too hard", they can have really detailed insight into the project at every level, if they choose. From project managers that are answerable to them alone. Apple really likes (and benefits from) being able to steer their own ship, self-determination is huge to them.
